Squashed 'sources/cxx-stl/llvm-libc++abi/libcxxabi/' changes from 0fd027b..2b683fb

2b683fb Let libc++abi compile with gcc.

git-subtree-dir: sources/cxx-stl/llvm-libc++abi/libcxxabi
git-subtree-split: 2b683fb0b15d8a56537640c5c9d8073da55e5871
Merge commit 'b7e718ef59213cef3c7c19047d8ef1a892c4f6c4' into upstream-llvm