[libcxx][CI] Use lld for everything in the ARM picolib builds

Our host compiler is a clang install that will default to ld if
not told otherwise.

We were telling meson to use lld, but the way that we did it
was outdated, which lead to picolib producing a linker script
that lld could not use.

Using the `c_ld` setting fixes this problem. See:
https://mesonbuild.com/Machine-files.html#binaries

Then to use lld in tests we need `-fuse-ld=lld` in the config
files.

Author: DavidSpickett

libcxx/cmake/caches/Armv7M-picolibc.cmake

@@ -5,6 +5,7 @@ set(CMAKE_C_COMPILER_TARGET "armv7m-none-eabi" CACHE STRING "")
 set(CMAKE_C_FLAGS "-mfloat-abi=soft" CACHE STRING "")
 set(CMAKE_SYSTEM_NAME Generic CACHE STRING "")
 set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY CACHE STRING "")
+set(LLVM_USE_LINKER "lld" CACHE STRING "")
 set(COMPILER_RT_BAREMETAL_BUILD ON CACHE BOOL "")
 set(COMPILER_RT_BUILD_LIBFUZZER OFF CACHE BOOL "")
 set(COMPILER_RT_BUILD_PROFILE OFF CACHE BOOL "")

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

@@ -13,7 +13,7 @@ config.substitutions.append(('%{compile_flags}',
     ' -Wno-atomic-alignment'
 ))
 config.substitutions.append(('%{link_flags}',
-    '-nostdlib -nostdlib++ -L %{lib-dir} -lc++ -lc++abi'
+    '-fuse-ld=lld -nostdlib -nostdlib++ -L %{lib-dir} -lc++ -lc++abi'
     ' -lc -lm -lclang_rt.builtins -lsemihost -lcrt0-semihost' +
     ' -T {}'.format(libc_linker_script) +
     ' -Wl,--defsym=__flash=0x0'

libcxx/utils/ci/build-picolibc.sh

@@ -81,7 +81,7 @@ cat <<EOF > "${picolibc_build_dir}/meson-cross-build.txt"
 c = ['${CC:-cc}', '--target=${target}', '-mfloat-abi=soft', '-nostdlib']
 ar = 'llvm-ar'
 as = 'llvm-as'
-ld = 'lld'
+c_ld = 'lld'
 strip = 'llvm-strip'
 [host_machine]
 system = 'none'

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

@@ -8,7 +8,7 @@ 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'
 ))
 config.substitutions.append(('%{link_flags}',
-    '-nostdlib -nostdlib++ -L %{lib} -lc++ -lc++abi'
+    '-fuse-ld=lld -nostdlib -nostdlib++ -L %{lib} -lc++ -lc++abi'
     ' -lc -lm -lclang_rt.builtins -lsemihost -lcrt0-semihost' +
     ' -T {}'.format(libc_linker_script) +
     ' -Wl,--defsym=__flash=0x0'

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

@@ -8,7 +8,7 @@ config.substitutions.append(('%{compile_flags}',
     '-nostdinc++ -I %{include}'
 ))
 config.substitutions.append(('%{link_flags}',
-    '-nostdlib -nostdlib++ -L %{lib} -lunwind'
+    '-fuse-ld=lld -nostdlib -nostdlib++ -L %{lib} -lunwind'
     ' -lc -lm -lclang_rt.builtins -lsemihost -lcrt0-semihost' +
     ' -T {}'.format(libc_linker_script) +
     ' -Wl,--defsym=__flash=0x0'