[CMake][Fuchsia] Build libclang_rt.builtins for arm-fuchsia (#166686)

No other runtimes can yet be built for the arm-fuchsia target,
but this one can be.  There is no OS-specific code in the arm
builtins needed for Fuchsia.

Author: frobtech

clang/cmake/caches/Fuchsia-stage2.cmake

@@ -200,16 +200,17 @@ endforeach()
 
 if(FUCHSIA_SDK)
   set(FUCHSIA_aarch64-unknown-fuchsia_NAME arm64)
+  set(FUCHSIA_arm-unknown-fuchsia_NAME arm)
   set(FUCHSIA_i386-unknown-fuchsia_NAME x64)
   set(FUCHSIA_x86_64-unknown-fuchsia_NAME x64)
   set(FUCHSIA_riscv64-unknown-fuchsia_NAME riscv64)
-  foreach(target i386-unknown-fuchsia;x86_64-unknown-fuchsia;aarch64-unknown-fuchsia;riscv64-unknown-fuchsia)
+  foreach(target i386-unknown-fuchsia;x86_64-unknown-fuchsia;aarch64-unknown-fuchsia;arm-unknown-fuchsia;riscv64-unknown-fuchsia)
     set(FUCHSIA_${target}_COMPILER_FLAGS "--target=${target} -I${FUCHSIA_SDK}/pkg/sync/include -I${FUCHSIA_SDK}/pkg/fdio/include")
     set(FUCHSIA_${target}_LINKER_FLAGS "-L${FUCHSIA_SDK}/arch/${FUCHSIA_${target}_NAME}/lib")
     set(FUCHSIA_${target}_SYSROOT "${FUCHSIA_SDK}/arch/${FUCHSIA_${target}_NAME}/sysroot")
   endforeach()
 
-  foreach(target i386-unknown-fuchsia;x86_64-unknown-fuchsia;aarch64-unknown-fuchsia;riscv64-unknown-fuchsia)
+  foreach(target i386-unknown-fuchsia;x86_64-unknown-fuchsia;aarch64-unknown-fuchsia;arm-unknown-fuchsia;riscv64-unknown-fuchsia)
     # Set the per-target builtins options.
     list(APPEND BUILTIN_TARGETS "${target}")
     set(BUILTINS_${target}_CMAKE_SYSTEM_NAME Fuchsia CACHE STRING "")