[LLDB][MIPS] Check if libatomic needs to be specified explicitly

Patch by Nitesh Jain.

Summary : The problem appears while linking liblldb.so. The class Address contain atomic variable m_offset. The loading and storing of variable is access via atomic_load_8 and atomic_store_8 functions. Some target fail to implicitly link libatomic, which cause undefine reference to atomic_store_8/atomic_load_8. This patch uses http://reviews.llvm.org/D20896 to check if libatomic need to be explicitly link.

Reviewed by labath.
Differential: D20464

llvm-svn: 274121