Basic signal tests: Can we deliver a signal?

Basic signal tests that tests can we deliver a signal via raise() and
can we deliver one via SIGALARM asynchronously.

In addition, tests whether or not on ARM T32 (Thumb) code can interrupt
A32 (normal) and vice versa.

While this test is aimed at ensuring basic qemu signals are working,
it's good to have in the base.

Sponsored by: Netflix
Discussed with: kevans, cognet
Differential Revision: https://reviews.freebsd.org/D33078