[CMake][libcxx] Use builtins rather than gcc_s when compiler-rt is requested

When compiler-rt is requested, we should attempt to link compiler-rt
builtins library rather than gcc_s.

Differential Revision: https://reviews.llvm.org/D31617

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@299599 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed