[libc++abi] Use libgcc and libgcc_s to provide _Unwind symbols instead of libgcc_eh.a

Summary:
libgcc_eh.a cannot be used when building libc++abi as a shared library (the default configuration). See this post for some more discussion: https://gcc.gnu.org/ml/gcc/2012-03/msg00104.html

This patch reverts back to using libgcc_s when linking libc++abi.so. 


Reviewers: danalbert, chandlerc, mclow.lists, ismail, compnerd

Subscribers: vkalintiris, cfe-commits

Differential Revision: http://reviews.llvm.org/D15440

llvm-svn: 255559