kdump: simplify/remove per-arch #ifdefs

It is acceptable for syscallabi to map SV_ABI to SYSDECODE_ABI on all
architectures; libsysdecode will return not-found sentinel values if
it does not have a syscall name or errno mapping for a given
architecture.

Also, use LP64 for the SV_ILP32 -> SYSDECODE_ABI_LINUX32 mapping,
for any future 32- on 64-bit linuxulator implementation.

Reviewed by: jhb
Sponsored by: Turing Robotic Industries Inc.