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.