[Fuchsia] Include baremetal ARM builtins and libc (#83949)

For now we only include the armv6m-unknown-eabi target but we plan to
include more targets in the future (including multilibs).

clang/cmake/caches/Fuchsia-stage2.cmake

@@ -300,6 +300,39 @@ if(FUCHSIA_SDK)
   set(LLVM_RUNTIME_MULTILIB_hwasan+noexcept_TARGETS "aarch64-unknown-fuchsia;riscv64-unknown-fuchsia" CACHE STRING "")
 endif()
 
+foreach(target armv6m-unknown-eabi)
+  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_${target}_CMAKE_BUILD_TYPE RelWithDebInfo CACHE STRING "")
+  foreach(lang C;CXX;ASM)
+    set(BUILTINS_${target}_CMAKE_${lang}_FLAGS "--target=${target} -mcpu=cortex-m0plus -mthumb" CACHE STRING "")
+  endforeach()
+  foreach(type SHARED;MODULE;EXE)
+    set(BUILTINS_${target}_CMAKE_${type}_LINKER_FLAGS "-fuse-ld=lld" CACHE STRING "")
+  endforeach()
+  set(BUILTINS_${target}_COMPILER_RT_BAREMETAL_BUILD ON CACHE BOOL "")
+
+  list(APPEND RUNTIME_TARGETS "${target}")
+  set(RUNTIMES_${target}_CMAKE_SYSTEM_NAME Generic CACHE STRING "")
+  set(RUNTIMES_${target}_CMAKE_SYSTEM_PROCESSOR arm CACHE STRING "")
+  set(RUNTIMES_${target}_CMAKE_SYSROOT "" CACHE STRING "")
+  set(RUNTIMES_${target}_CMAKE_BUILD_TYPE RelWithDebInfo CACHE STRING "")
+  set(RUNTIMES_${target}_CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY CACHE STRING "")
+  foreach(lang C;CXX;ASM)
+    set(RUNTIMES_${target}_CMAKE_${lang}_FLAGS "--target=${target} -mcpu=cortex-m0plus -mthumb" CACHE STRING "")
+  endforeach()
+  foreach(type SHARED;MODULE;EXE)
+    set(RUNTIMES_${target}_CMAKE_${type}_LINKER_FLAGS "-fuse-ld=lld" CACHE STRING "")
+  endforeach()
+  set(RUNTIMES_${target}_LLVM_LIBC_FULL_BUILD ON CACHE BOOL "")
+  set(RUNTIMES_${target}_LIBC_ENABLE_USE_BY_CLANG ON CACHE BOOL "")
+  set(RUNTIMES_${target}_LLVM_INCLUDE_TESTS OFF CACHE BOOL "")
+  set(RUNTIMES_${target}_LLVM_ENABLE_ASSERTIONS OFF CACHE BOOL "")
+  set(RUNTIMES_${target}_LLVM_ENABLE_RUNTIMES "libc" CACHE STRING "")
+endforeach()
+
 foreach(target riscv32-unknown-elf)
   list(APPEND BUILTIN_TARGETS "${target}")
   set(BUILTINS_${target}_CMAKE_SYSTEM_NAME Generic CACHE STRING "")