[libc] fix lfind entrypoints (#115771)

- move arm entrypoint to fullbuild only
- remove baremetal entrypoints; we avoid POSIX on baremetal
- remove darwin/arm and windows entrypoints since these are untested

Fixes: #114692
GitOrigin-RevId: f1800df5d53e14e2d865273e42bb0a3968129a7f
5 files changed