sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

Use standard pattern for stdargs.h

We don't support older compilers. Most of the code in these files is
for pre-3.0 gcc, which is at least 15 years obsolete. Move to using
phk's sys/_stdargs.h for all these platforms.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D14323

Import RISC-V machine headers. This is a minimal set required to compile
kernel and userland.

Reviewed by: andrew, imp, kib
Sponsored by: DARPA, AFRL
Sponsored by: HEIF5
Differential Revision: https://reviews.freebsd.org/D4554