Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Account for <machine/fsr.h> no longer being usable for assembler code.

Add jmpbuf offsets. Remove ucontext offsets.

Catch up to jmpbuf changes. Define offsets used from assmebly language
in another file so that it is easy to see what they are and keep them in
sync with the headers (grafted from the kernel generated assym.s).