arm64: Raise a SIGILL if we fail reading an insn

When reading an instruction we may fail if another thread is unmapping
a page while the current thread is executing from it. In this case
raise a SIGILL rather than panicing the kernel.

Reviewed by: harry.moulton_arm.com
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D50203