[CMake][Fuchsia] Enable standalone libatomic

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.

Differential Revision: https://reviews.llvm.org/D151681

clang/cmake/caches/Fuchsia-stage2.cmake

@@ -143,6 +143,7 @@ foreach(target aarch64-unknown-linux-gnu;armv7-unknown-linux-gnueabihf;i386-unkn
     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 "${target}")
@@ -159,6 +160,7 @@ foreach(target aarch64-unknown-linux-gnu;armv7-unknown-linux-gnueabihf;i386-unkn
     set(RUNTIMES_${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 "")