[libcxxabi] Fewer assumptions about path from libcxx to libcxxabi

This is useful for projects that pull in libcxx and libcxxabi and build
them using out-of-tree build files, but don't make them sibling
directories (or don't call the sibling directories libcxx and libcxxabi
for some reason).

Fixes PR49313.

Differential Revision: https://reviews.llvm.org/D97379