Remove $FreeBSD$: one-line .c pattern

Remove /^[\s*]*__FBSDID\("\$FreeBSD\$"\);?\s*\n/

csu: move common code to libc

Why? Most trivial point, it shaves around 600 bytes from the dynamic
binaries on amd64. Less trivial, the removed code is no longer part of
the ABI, and we can ship updates to it with libc updates. Right now most
of the csu is linked into the binaries and require us to do somewhat
tricky ABI compat when it needs to change. For instance, the init_array
change would be much simpler and does not require note tagging if we
have init calling code in libc.

This could be improved more, by splitting dynamic and static
initialization. For instance, &_DYNAMIC tests can be removed then.
Such change, nonetheless, would require building libc three times.
I left this for later, after this change stabilizes, if ever.

Reviewed by: markj
Discussed with: jrtc27 (some objections, see the review), imp
Tested by: markj (aarch64)
Sponsored by: The FreeBSD Foundation
MFC after: 3 weeks
Differential revision: https://reviews.freebsd.org/D37220

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