[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