[libcxxabi] Introduce an externally threaded libc++abi variant.

r281179 Introduced an externally threaded variant of the libc++ library. This
patch adds support for a similar library variant for libc++abi.

Differential revision: https://reviews.llvm.org/D27575

Reviewers: EricWF

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@290888 91177308-0d34-0410-b5e6-96231b3b80d8
13 files changed