[LLVM][MIPS] Introduce 64 bit atomic check in CheckAtomic.cmake

Patch by Nitesh Jain.

Summary: On some target like MIPS32 we need to explicitly link atomic library for 64 bit atomic operations. This module then can be used in LLDB (http://reviews.llvm.org/D20464) or Libcxx (http://reviews.llvm.org/D16613) for explicitly link to atomic library.

Reviewers: chandlerc, beanz
Differential: reviews.llvm.org/D20896
llvm-svn: 273534