arm64: Add a BTI landing pad to .mcount

The .mcount function needs a BTI branch target. As we can't rely on
asm.h being included use the hint version of a "bti c" instruction.
This is a nop when BTI is not supported or not used.

Reviewed by: markj
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D42230