Add infrastructure required for Linux coredump support

This adds sv_elf_core_osabi, sv_elf_core_abi_vendor,
and sv_elf_core_prepare_notes fields to struct sysentvec,
and modifies imgact_elf.c to make use of them instead
of hardcoding FreeBSD-specific values. It also updates all
of the ABI definitions to preserve current behaviour.

This makes it possible to implement non-native ELF coredump
support without unnecessary code duplication. It will be used
for Linux coredumps.

Reviewed By: kib
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D30921