[cmake][Fuchsia] armv8m -> armv8m.main (#98695)

armv8m builtins aren't being built because `armv8m` doesn't match any of
the arm cpu strings in compiler-rt cmake files. Instead there's
`armv8m.main` and `armv8m.base`. We want to use the `armv8m.main`
version.

Author: PiJoules

clang/cmake/caches/Fuchsia-stage2.cmake

@@ -300,14 +300,18 @@ if(FUCHSIA_SDK)
   set(LLVM_RUNTIME_MULTILIB_hwasan+noexcept_TARGETS "aarch64-unknown-fuchsia;riscv64-unknown-fuchsia" CACHE STRING "")
 endif()
 
-foreach(target armv6m-unknown-eabi;armv7m-unknown-eabi;armv8m-unknown-eabi)
+foreach(target armv6m-unknown-eabi;armv7m-unknown-eabi;armv8m.main-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} -mthumb" CACHE STRING "")
+    set(BUILTINS_${target}_CMAKE_${lang}_local_flags "--target=${target} -mthumb")
+    if(${target} STREQUAL "armv8m.main-unknown-eabi")
+      set(BUILTINS_${target}_CMAKE_${lang}_local_flags "${BUILTINS_${target}_CMAKE_${lang}_local_flags} -mfloat-abi=hard -march=armv8m.main+fp+dsp -mcpu=cortex-m33" CACHE STRING "")
+    endif()
+    set(BUILTINS_${target}_CMAKE_${lang}_FLAGS "${BUILTINS_${target}_CMAKE_${lang}_local_flags}" CACHE STRING "")
   endforeach()
   foreach(type SHARED;MODULE;EXE)
     set(BUILTINS_${target}_CMAKE_${type}_LINKER_FLAGS "-fuse-ld=lld" CACHE STRING "")