arm64: emulate swp/swpb instructions

Add another undefined instruction handler for compat32 and watch out for
SWP/SWPB instructions.

SWP/SWPB were deprecated in ARMv6 and declared obsolete in ARMv7, but
this implementation is motivated by some proprietary software that still
uses SWP/SWPB. Because it's deprecated, emulation is pushed back behind
a sysctl that defaults to OFF in GENERIC so that it doesn't potentially
adversely affect package builds; it's unknown whether software may test
for a functional swp/swpb instruction with the desire of using it later,
so we err on the side of caution to ensure we don't end up with swp/swpb
use in freebsd/arm packages (which are built on aarch64).

The EMUL_SWP config option may be used to enable emulation by default in
environments where emulation is desired and won't really be turned off.

Reviewed by: andrew, mmel (both earlier version)
Sponsored by: Stormshield
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D39667