Import NetBSD's version, which is perfectly fine.

Submitted by:	jmallet@
Remove asm.h dependency in a hackish way.
Fixed comment.
Don't include asm.h in non-asm files.
Remove #define to get kludges that asm.h used to define
Move clever macros to access assembler instructions to trap.c
Remove __ASSEMBLER__ ifdefs in regdef.h: they aren't needed anymore.
Add a historic footnote.