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

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

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