elf: add some definitions for i386 and amd64 relocations

I believe that rtld does not need to implement them, they are mostly for
the static linker. 'Mostly' because for amd64 our kernel linker loads
object files, and amd64 relocation types could be observed.

Defines were taken from glibc sources.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D28205