Teach DTrace about BTI on arm64

The Branch Target Identification (BTI) Armv8-A extension adds new
instructions that can be placed where we may indirrectly branch to,
e.g. at the start of a function called via a function pointer. We can't
emulate these in DTrace as the kernel will have raised a different
exception before the DTrace handler has run.

Skip over the BTI instruction if it's used as the first instruction in
a function.

Sponsored by: The FreeBSD Foundation