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

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

Add a <machine/fpu.h> for i386 that includes <machine/npx.h>.

arm64 has a similar wrapper. This permits defining <machine/fpu.h> as
the standard header for fpu_kern_*.

Reviewed by: kib
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D26753