builtins: implement __emutls_get_address

For targets like Android that depends on libgcc's emulated thread local storage.
clang/llvm can generate calls to __emutls_get_address since
http://reviews.llvm.org/D10522 and http://reviews.llvm.org/D10524

Differential Revision: http://reviews.llvm.org/D12001

llvm-svn: 246454