arm64: add swapueword8/32

Much like casueword*, except just a plain old swap. Maintains a similar
interface to casu(9)- return value -1 (fault), 0 (success), or 1 (fail),
and also both ll/sc and LSE variants are implemented.

These will be used to implement 32-bit swp/swpb emulation on aarch64.

Reveiwed by: andrew
Sponsored by: Stormshield
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D39837