Re-commit "Introduce a cmake module to figure out whether we need to link with libatomic."

This re-applies commit r260235. However, this time we add -gcc-toolchain
to the compiler's flags when the user has specified the LIBCXX_GCC_TOOLCHAIN
variable.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@260515 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed