[libc++][libunwind] Fixes to allow GCC 13 to compile libunwind/libc++abi/libc++

These are changes to allow GCC 13 to successfully compile the runtimes stack.

Reviewed By: ldionne, #libc, #libunwind, MaskRay

Spies: MaskRay, zibi, SeanP, power-llvm-team, mstorsjo, arichardson, libcxx-commits

Differential Revision: https://reviews.llvm.org/D151387