Fix compile for -DLIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY

Summary: Compilation with  -DLIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY was failing due to missing declarations of functions used in libcxx/include/atomic. The lines this commit affects are the places where those functions are defined, now moved to be always defined.

Reviewers: #libc, ldionne

Reviewed By: #libc, ldionne

Subscribers: miyuki, dexonsmith, ldionne, jfb, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D80372