[libc] [libcxx] [llvm] Remove last few bits for Native Client support (PR #148983)

[Fangrui Song via llvm-commits]

https://github.com/MaskRay approved this pull request.

llvm-readobj change looks good. libc++ maintainers need to approve the libc++ part.

https://github.com/llvm/llvm-project/pull/148983