rtld: Teach rtld about the BTI elf note

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

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

(cherry picked from commit 5270cc9757fb4e0c1303ec44c2602b75acf3806d)