Introduce a cmake module to figure out whether we need to link with libatomic.

Summary:
This fixes the tests under std/atomics for 32-bit MIPS CPUs where the
8-byte atomic operations call into the libatomic library.

Reviewers: dsanders, mclow.lists, EricWF, jroelofs, joerg

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D16613

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@260235 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed