arm64: add tests for swp/swpb emulation

One test is suitable to be hooked up to the build, so I've done this
here. The other test lives in tools/regression because failure is a
bit more subjective -- generally, one runs it for some unbounded amount
of time and observe if it eventually exits because two threads acquired
the same mutex.

Reviewed by: imp, mmel
Sponsored by: Stormshield
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D39668