[libcxx] Add build/test support for the externally threaded libc++abi variant

Differential revision: https://reviews.llvm.org/D27576

Reviewers: EricWF

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@290889 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed