<sys/reg.h>: Add ELF32_REGSET.

This permits adding a register set for FREEBSD32 ABIs.

While here, include <sys/linker_set.h> to make this header more
self-contained in the kernel.

Reviewed by: andrew, markj
Sponsored by: The University of Cambridge, Google, Inc.
Differential Revision: https://reviews.freebsd.org/D34536