Add BTI exceptions

[andrew]

Summary

We could hit these when executing code marked as using BTI but jumps
to a non-branch target instruction.

Sponsored by: Arm Ltd
Sponsored by: The FreeBSD Foundation

[emaste]

sys/arm64/arm64/trap.c

708

it's too bad there's not something like ILL_ILLBRANCH or such, but it looks like ILL_ILLOPC is consistent with Linux

[kib]

sys/arm64/arm64/trap.c

708

Nothing prevents us from adding specific si_value if it is useful.

[imp]

This looks good to me... Will have to remember to check qemu's bsd-user to add this.

[andrew]

sys/arm64/arm64/trap.c

708

We set si_trapno to the exception number if userspace needs to know it's a BTI exception.