libc++abi: work around layering violation

This papers over a layering violation currently between libc++abi and libunwind.
It reaches into the sources to get the declaration of an ABI defined function.
This should allow the ARM buildbot to continue building libc++abi again.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@235965 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed