[libunwind][mips] Include gcc_s for linkage

When compiling with optimizations, mips requires various helper routines(__ashldi3 and the like) contained in libgcc_s.
Conditionally include libgcc_s in the set of libraries to be linked to.

Differential Revision: https://reviews.llvm.org/D50243


git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@339878 91177308-0d34-0410-b5e6-96231b3b80d8