Remove the sed hack for ABI tag notes.

The ELF notes compiled in C were placed in a section with the wrong type
(SHT_PROGBITS instead of SHT_NOTE). Previously, sed was used on the
generated assembly to rewrite the section type. Instead, write the notes
in assembly which permits setting the correct section type directly.

While here, move inline assembly entry points out of C and into assembly
for aarch64, arm, and riscv.

Reviewed by: kib
Tested on: amd64 (cirrus-ci), riscv64
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D25211