[CMake][Fuchsia] Enable standalone libatomic

[abrachet]

Summary

BUILTINS_${target}_COMPILER_RT_BUILD_STANDALONE_LIBATOMIC
actually builds libatomic, and
RUNTIMES_${target}_COMPILER_RT_BUILD_STANDALONE_LIBATOMIC
tells the compiler-rt tests that we built it and it is
safe to use in tests.

[phosek]

LGTM

clang/cmake/caches/Fuchsia-stage2.cmake

if(LINUX_${target}_SYSROOT)
     set(BUILTINS_${target}_CMAKE_BUILD_TYPE RelWithDebInfo CACHE STRING "")
     set(BUILTINS_${target}_CMAKE_C_FLAGS "--target=${target}" CACHE STRING "")
     set(BUILTINS_${target}_CMAKE_CXX_FLAGS "--target=${target}" CACHE STRING "")
     set(BUILTINS_${target}_CMAKE_ASM_FLAGS "--target=${target}" CACHE STRING "")
     set(BUILTINS_${target}_CMAKE_SYSROOT ${LINUX_${target}_SYSROOT} CACHE STRING "")
     set(BUILTINS_${target}_CMAKE_SHARED_LINKER_FLAGS "-fuse-ld=lld" CACHE STRING "")
     set(BUILTINS_${target}_CMAKE_MODULE_LINKER_FLAGS "-fuse-ld=lld" CACHE STRING "")
     set(BUILTINS_${target}_CMAKE_EXE_LINKER_FLAG "-fuse-ld=lld" CACHE STRING "")
+    set(BUILTINS_${target}_COMPILER_RT_BUILD_STANDALONE_LIBATOMIC ON CACHE BOOL "")
 
     # Set the per-target runtimes options.
     list(APPEND RUNTIME_TARGETS "${target}")
     set(RUNTIMES_${target}_CMAKE_SYSTEM_NAME Linux CACHE STRING "")
     set(RUNTIMES_${target}_CMAKE_BUILD_TYPE RelWithDebInfo CACHE STRING "")
     set(RUNTIMES_${target}_CMAKE_C_FLAGS "--target=${target}" CACHE STRING "")
     set(RUNTIMES_${target}_CMAKE_CXX_FLAGS "--target=${target}" CACHE STRING "")
     set(RUNTIMES_${target}_CMAKE_ASM_FLAGS "--target=${target}" CACHE STRING "")
     set(RUNTIMES_${target}_CMAKE_SYSROOT ${LINUX_${target}_SYSROOT} CACHE STRING "")
     set(RUNTIMES_${target}_CMAKE_SHARED_LINKER_FLAGS "-fuse-ld=lld" CACHE STRING "")
     set(RUNTIMES_${target}_CMAKE_MODULE_LINKER_FLAGS "-fuse-ld=lld" CACHE STRING "")
     set(RUNTIMES_${target}_CMAKE_EXE_LINKER_FLAGS "-fuse-ld=lld" CACHE STRING "")
     set(RUNTIMES_${target}_COMPILER_RT_CXX_LIBRARY "libcxx" CACHE STRING "")
     set(RUNTIMES_${target}_COMPILER_RT_USE_BUILTINS_LIBRARY ON CACHE BOOL "")
     set(RUNTIMES_${target}_COMPILER_RT_USE_LLVM_UNWINDER ON CACHE BOOL "")
     set(RUNTIMES_${target}_COMPILER_RT_CAN_EXECUTE_TESTS ON CACHE BOOL "")
+    set(RUNTIMES_${target}_COMPILER_RT_BUILD_STANDALONE_LIBATOMIC ON CACHE BOOL "")
     set(RUNTIMES_${target}_LIBUNWIND_ENABLE_SHARED OFF CACHE BOOL "")
     set(RUNTIMES_${target}_LIBUNWIND_USE_COMPILER_RT ON CACHE BOOL "")
     set(RUNTIMES_${target}_LIBCXXABI_USE_COMPILER_RT ON CACHE BOOL "")
     set(RUNTIMES_${target}_LIBCXXABI_ENABLE_SHARED OFF CACHE BOOL "")
     set(RUNTIMES_${target}_LIBCXXABI_USE_LLVM_UNWINDER ON CACHE BOOL "")
     set(RUNTIMES_${target}_LIBCXXABI_INSTALL_LIBRARY OFF CACHE BOOL "")
     set(RUNTIMES_${target}_LIBCXX_USE_COMPILER_RT ON CACHE BOOL "")
     set(RUNTIMES_${target}_LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")