[libcxx][CI] Prepare ARM Picolib builds for upgrade to clang 21

Some time between Clang 19 and Clang 21, changes were made to the
bare metal toolchain driver, and on top of that we (Linaro) updated
our host OS to Ubuntu Jammy. This has caused the tests to not
build with clang-21.

1. The host version of ld (2.38) not being able to parse certain
   DWARFv5 constructs.

Normally I would switch to lld instead. However when I did this,
I found that Picolib's ARM linker script uses ALIGN_WITH_INPUT,
which lld does not support.

So instead I'm sticking with ld and falling back to DWARFv4.
Until such time as we upgrade the host to Ubuntu Noble.

(if you're wondering how we can use ld in the first place, given
the host is AArch64, the AArch64 ld is also capable of linking
ARM 32-bit binaries)

2. Many missing symbol errors.

This is solved in two parts:
* Adding libunwind to the testing linker flags.
* Putting the libraries in a group so that the ordering does
  not matter.

And finally for exception_guard.odr.sh.cpp, I have moved the
object files to before the linker flags. ld is sensitive to
the order of objects on the command line, and this should not
break other configurations.

I've tested these with 19.1.7 too so my plan is to land these
changes first, then proceed with upgrading to 21.x.

Author: DavidSpickett

libcxx/cmake/caches/Armv7M-picolibc.cmake

@@ -1,8 +1,8 @@
 set(CMAKE_ASM_COMPILER_TARGET "armv7m-none-eabi" CACHE STRING "")
 set(CMAKE_CXX_COMPILER_TARGET "armv7m-none-eabi" CACHE STRING "")
-set(CMAKE_CXX_FLAGS "-mfloat-abi=soft" CACHE STRING "")
+set(CMAKE_CXX_FLAGS "-mfloat-abi=soft -gdwarf-4" CACHE STRING "")
 set(CMAKE_C_COMPILER_TARGET "armv7m-none-eabi" CACHE STRING "")
-set(CMAKE_C_FLAGS "-mfloat-abi=soft" CACHE STRING "")
+set(CMAKE_C_FLAGS "-mfloat-abi=soft -gdwarf-4" CACHE STRING "")
 set(CMAKE_SYSTEM_NAME Generic CACHE STRING "")
 set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY CACHE STRING "")
 set(COMPILER_RT_BAREMETAL_BUILD ON CACHE BOOL "")

libcxx/test/configs/armv7m-picolibc-libc++.cfg.in

@@ -5,16 +5,16 @@ libc_linker_script = '@CMAKE_INSTALL_PREFIX@/lib/picolibcpp.ld'
 config.substitutions.append(('%{flags}', '--sysroot=@CMAKE_INSTALL_PREFIX@'))
 
 config.substitutions.append(('%{compile_flags}',
-    '-nostdinc++ -I %{include-dir} -I %{target-include-dir} -I %{libcxx-dir}/test/support'
+    '-nostdinc++ -I %{include-dir} -I %{target-include-dir} -I %{libcxx-dir}/test/support -gdwarf-4'
 
     # Disable warnings in the <atomic> implementation:
     # "large atomic operation may incur significant performance penalty; the
     # access size (4 bytes) exceeds the max lock-free size (0 bytes)"
     ' -Wno-atomic-alignment'
 ))
 config.substitutions.append(('%{link_flags}',
-    '-nostdlib -nostdlib++ -L %{lib-dir} -lc++ -lc++abi'
-    ' -lc -lm -lclang_rt.builtins -lsemihost -lcrt0-semihost' +
+    '-nostdlib -nostdlib++ -L %{lib-dir} -Wl,--start-group -lc++ -lc++abi'
+    ' -lunwind -lc -lm -lclang_rt.builtins -lsemihost -lcrt0-semihost -Wl,--end-group' +
     ' -T {}'.format(libc_linker_script) +
     ' -Wl,--defsym=__flash=0x0'
     ' -Wl,--defsym=__flash_size=0x400000'

libcxx/test/libcxx/utilities/exception_guard.odr.sh.cpp

@@ -14,7 +14,7 @@
 
 // RUN: %{cxx} %s %{flags} %{compile_flags} -c -o %t.except.o   -O1 -fexceptions
 // RUN: %{cxx} %s %{flags} %{compile_flags} -c -o %t.noexcept.o -O1 -fno-exceptions
-// RUN: %{cxx} %{flags} %{link_flags} -o %t.exe %t.except.o %t.noexcept.o
+// RUN: %{cxx} %{flags} -o %t.exe %t.except.o %t.noexcept.o %{link_flags}
 // RUN: %{run}
 
 #include <__utility/exception_guard.h>

libcxx/utils/ci/build-picolibc.sh

@@ -78,7 +78,7 @@ rm -rf "${picolibc_source_dir}/picolibc-${picolibc_commit}"
 
 cat <<EOF > "${picolibc_build_dir}/meson-cross-build.txt"
 [binaries]
-c = ['${CC:-cc}', '--target=${target}', '-mfloat-abi=soft', '-nostdlib']
+c = ['${CC:-cc}', '--target=${target}', '-mfloat-abi=soft', '-nostdlib', '-gdwarf-4']
 ar = 'llvm-ar'
 as = 'llvm-as'
 ld = 'lld'

libcxxabi/test/configs/armv7m-picolibc-libc++abi.cfg.in

@@ -5,11 +5,11 @@ libc_linker_script = '@CMAKE_INSTALL_PREFIX@/lib/picolibcpp.ld'
 config.substitutions.append(('%{flags}', '--sysroot=@CMAKE_INSTALL_PREFIX@'))
 
 config.substitutions.append(('%{compile_flags}',
-    '-nostdinc++ -I %{include} -I %{cxx-include} -I %{cxx-target-include} %{maybe-include-libunwind} -I %{libcxx}/test/support -I %{libcxx}/src -D_LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS'
+    '-nostdinc++ -I %{include} -I %{cxx-include} -I %{cxx-target-include} %{maybe-include-libunwind} -I %{libcxx}/test/support -I %{libcxx}/src -D_LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS -gdwarf-4'
 ))
 config.substitutions.append(('%{link_flags}',
-    '-nostdlib -nostdlib++ -L %{lib} -lc++ -lc++abi'
-    ' -lc -lm -lclang_rt.builtins -lsemihost -lcrt0-semihost' +
+    '-nostdlib -nostdlib++ -L %{lib} -Wl,--start-group -lc++ -lc++abi'
+    ' -lunwind -lc -lm -lclang_rt.builtins -lsemihost -lcrt0-semihost -Wl,--end-group' +
     ' -T {}'.format(libc_linker_script) +
     ' -Wl,--defsym=__flash=0x0'
     ' -Wl,--defsym=__flash_size=0x400000'

libunwind/test/configs/armv7m-picolibc-libunwind.cfg.in

@@ -5,11 +5,11 @@ libc_linker_script = '@CMAKE_INSTALL_PREFIX@/lib/picolibcpp.ld'
 config.substitutions.append(('%{flags}', '--sysroot=@CMAKE_INSTALL_PREFIX@'))
 
 config.substitutions.append(('%{compile_flags}',
-    '-nostdinc++ -I %{include}'
+    '-nostdinc++ -I %{include} -gdwarf-4'
 ))
 config.substitutions.append(('%{link_flags}',
-    '-nostdlib -nostdlib++ -L %{lib} -lunwind'
-    ' -lc -lm -lclang_rt.builtins -lsemihost -lcrt0-semihost' +
+    '-nostdlib -nostdlib++ -L %{lib} -Wl,--start-group -lunwind'
+    ' -lc -lm -lclang_rt.builtins -lsemihost -lcrt0-semihost -Wl,--end-group' +
     ' -T {}'.format(libc_linker_script) +
     ' -Wl,--defsym=__flash=0x0'
     ' -Wl,--defsym=__flash_size=0x400000'