csu: Teach csu about PAC and BTI

Add the Branch Target Identification (BTI) note to libc assembly
sources and Pointer Authentication Code (PAC) instructions to _init and
_fini.

_init and _fini may be called indirectly so need a BTI landing pad. As
they are non-leaf functions use the appropriate PAC instruction that
also guards against changing the link register.

As all object files need the note for any binary using these object files
we need to insert it in all asm files.

Reviewed by: markj
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D42227