[libcxx] Support the use of compiler-rt in lit tests

Don't link tests against libgcc when compiler-rt is being used.

Differential Revision: https://reviews.llvm.org/D40513

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@322044 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed