Minor libc++abi changes to make things build better with gcc.


git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@211748 91177308-0d34-0410-b5e6-96231b3b80d8
Fix building for iOS

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@211751 91177308-0d34-0410-b5e6-96231b3b80d8