[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
7 files changed