arm64: Support BTI checking in most of the kernel

LLD has the -zbti-report=error argument to check if the BTI note is
present when linking. To allow for this to be used when linking the
kernel and modules:
- Add the BTI note to the remaining assembly files
- Mark ptrauth.c as protected by BTI
- Disable -zbti-report for vmm hypervisor switching code as it's not
used there.

The linux64 module doesn't build with the flag as it includes vdso code
that doesn't include the note.

Reviewed by: imp, kib, emaste
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D45466

Import an optimized str{n}cmp on arm64

These are from the Arm Optimized Routines and don't use the VFP so are
safe to use in the kernel.

Sponsored by: The FreeBSD Foundation