[compiler-rt] [builtins] Don't build __atomic_* by default.

The locks need to be implemented in a shared library to work correctly,
so they shouldn't be part of libclang_rt.builtins.a, except in
specialized scenarios where the user can prove it will only be linked
once.

Differential Revision: https://reviews.llvm.org/D47606

llvm-svn: 334779