[libc] Support baremetal libc on aarch64 (#118691)

We have users that target baremetal aarch64.

Author: petrhosek

clang/cmake/caches/Fuchsia-stage2.cmake

@@ -300,19 +300,22 @@ if(FUCHSIA_SDK)
   set(LLVM_RUNTIME_MULTILIB_hwasan+noexcept_TARGETS "aarch64-unknown-fuchsia;riscv64-unknown-fuchsia" CACHE STRING "")
 endif()
 
-foreach(target armv6m-none-eabi;armv7m-none-eabi;armv8m.main-none-eabi;armv8.1m.main-none-eabi)
+foreach(target armv6m-none-eabi;armv7m-none-eabi;armv8m.main-none-eabi;armv8.1m.main-none-eabi;aarch64-none-elf)
   list(APPEND BUILTIN_TARGETS "${target}")
   set(BUILTINS_${target}_CMAKE_SYSTEM_NAME Generic CACHE STRING "")
   set(BUILTINS_${target}_CMAKE_SYSTEM_PROCESSOR arm CACHE STRING "")
   set(BUILTINS_${target}_CMAKE_SYSROOT "" CACHE STRING "")
   set(BUILTINS_${target}_CMAKE_BUILD_TYPE MinSizeRel CACHE STRING "")
   foreach(lang C;CXX;ASM)
-    set(BUILTINS_${target}_CMAKE_${lang}_local_flags "--target=${target} -mthumb")
+    set(BUILTINS_${target}_CMAKE_${lang}_local_flags "--target=${target}")
+    if(NOT ${target} STREQUAL "aarch64-none-elf")
+      set(BUILTINS_${target}_CMAKE_${lang}_local_flags "${BUILTINS_${target}_CMAKE_${lang}_local_flags} -mthumb")
+    endif()
     if(${target} STREQUAL "armv8m.main-none-eabi")
-      set(BUILTINS_${target}_CMAKE_${lang}_local_flags "${BUILTINS_${target}_CMAKE_${lang}_local_flags} -mfloat-abi=softfp -march=armv8m.main+fp+dsp -mcpu=cortex-m33" CACHE STRING "")
+      set(BUILTINS_${target}_CMAKE_${lang}_local_flags "${BUILTINS_${target}_CMAKE_${lang}_local_flags} -mfloat-abi=softfp -march=armv8m.main+fp+dsp -mcpu=cortex-m33")
     endif()
     if(${target} STREQUAL "armv8.1m.main-none-eabi")
-      set(BUILTINS_${target}_CMAKE_${lang}_local_flags "${BUILTINS_${target}_CMAKE_${lang}_local_flags} -mfloat-abi=hard -march=armv8.1-m.main+mve.fp+fp.dp -mcpu=cortex-m55" CACHE STRING "")
+      set(BUILTINS_${target}_CMAKE_${lang}_local_flags "${BUILTINS_${target}_CMAKE_${lang}_local_flags} -mfloat-abi=hard -march=armv8.1-m.main+mve.fp+fp.dp -mcpu=cortex-m55")
     endif()
     set(BUILTINS_${target}_CMAKE_${lang}_FLAGS "${BUILTINS_${target}_CMAKE_${lang}_local_flags}" CACHE STRING "")
   endforeach()
@@ -330,12 +333,15 @@ foreach(target armv6m-none-eabi;armv7m-none-eabi;armv8m.main-none-eabi;armv8.1m.
   foreach(lang C;CXX;ASM)
     # TODO: The preprocessor defines workaround various issues in libc and libc++ integration.
     # These should be addressed and removed over time.
-    set(RUNTIMES_${target}_CMAKE_${lang}_local_flags "--target=${target} -mthumb -Wno-atomic-alignment \"-Dvfprintf(stream, format, vlist)=vprintf(format, vlist)\" \"-Dfprintf(stream, format, ...)=printf(format)\" \"-Dgettimeofday(tv, tz)\" -D_LIBCPP_PRINT=1")
+    set(RUNTIMES_${target}_CMAKE_${lang}_local_flags "--target=${target} -Wno-atomic-alignment \"-Dvfprintf(stream, format, vlist)=vprintf(format, vlist)\" \"-Dfprintf(stream, format, ...)=printf(format)\" \"-Dgettimeofday(tv, tz)\" -D_LIBCPP_PRINT=1")
+    if(NOT ${target} STREQUAL "aarch64-none-elf")
+      set(RUNTIMES_${target}_CMAKE_${lang}_local_flags "${RUNTIMES_${target}_CMAKE_${lang}_local_flags} -mthumb")
+    endif()
     if(${target} STREQUAL "armv8m.main-none-eabi")
-      set(RUNTIMES_${target}_CMAKE_${lang}_local_flags "${RUNTIMES_${target}_CMAKE_${lang}_local_flags} -mfloat-abi=softfp -march=armv8m.main+fp+dsp -mcpu=cortex-m33" CACHE STRING "")
+      set(RUNTIMES_${target}_CMAKE_${lang}_local_flags "${RUNTIMES_${target}_CMAKE_${lang}_local_flags} -mfloat-abi=softfp -march=armv8m.main+fp+dsp -mcpu=cortex-m33")
     endif()
     if(${target} STREQUAL "armv8.1m.main-none-eabi")
-      set(RUNTIMES_${target}_CMAKE_${lang}_local_flags "${RUNTIMES_${target}_CMAKE_${lang}_local_flags} -mfloat-abi=hard -march=armv8.1-m.main+mve.fp+fp.dp -mcpu=cortex-m55" CACHE STRING "")
+      set(RUNTIMES_${target}_CMAKE_${lang}_local_flags "${RUNTIMES_${target}_CMAKE_${lang}_local_flags} -mfloat-abi=hard -march=armv8.1-m.main+mve.fp+fp.dp -mcpu=cortex-m55")
     endif()
     set(RUNTIMES_${target}_CMAKE_${lang}_FLAGS "${RUNTIMES_${target}_CMAKE_${lang}_local_flags}" CACHE STRING "")
   endforeach()