[libc][hdr] Add poll and socket overlay headers for GCC overlay builds. (#227961)

When building LLVM libc in overlay mode with gcc/g++, alias and macro
redirections in GNU libc headers (such as ppoll and socket functions)
cause conflict with libc header declarations.

Fixes in this PR:
- Add `poll_overlay.h` and `sys_socket_overlay.h` to wrap system headers
before libc macros and type declarations in overlay mode.
- Update proxy headers in `libc/hdr/` and `libc/hdr/types/`.
- Update CMake and Bazel build files accordingly.

Assisted-by: Gemini
GitOrigin-RevId: a72683f5a2bf63c8ee6dfb3c510f298589db08fe