[libc] Move generated elf header to include (#211428) The fact that `hdr/elf_proxy.h` was generated with headergen was causing dependency issues. This shouldn't be necessary, and this PR moves it out into `include` to avoid needing headergen for an overlay build. Assisted-by: Automated tooling, human reviewed. GitOrigin-RevId: 90010326302a92850b7a7ede1e0f9c5f169bad96