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

test/libcxx/test/target_info.py

@@ -172,6 +172,9 @@ class LinuxLocalTI(DefaultTargetInfo):
             flags += ['-lunwind', '-ldl']
         else:
             flags += ['-lgcc_s', '-lgcc']
+        use_libatomic = self.full_config.get_lit_bool('use_libatomic', False)
+        if use_libatomic:
+            flags += ['-latomic']
         san = self.full_config.get_lit_conf('use_sanitizer', '').strip()
         if san:
             # The libraries and their order are taken from the