libc: Teach libc about the BTI elf note

Add the Branch Target Identification (BTI) note to libc assembly
sources. As all obect files need the note for the library to have it
we need to insert it in all asm files.

Reviewed by: emaste, markj
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D42228

(cherry picked from commit fd5aaf2ea0178b03aa93c35245053247e5d3840c)