support for AVX on i386.
- Similar to amd64, move the FPU save area out of the PCB and instead
store saved FPU state in a variable-sized buffer after the PCB on the
stack.
- To support the variable PCB location, alter the locore code to only use
the bottom-most page of proc0stack for init386(). init386() returns
the correct stack pointer to locore which adjusts the stack for thread0
before calling mi_startup().
- Don't bother setting cr3 in thread0's pcb in locore before calling
init386(). It wasn't used (init386() overwrote it at the end) and
it doesn't work with the variable-sized FPU save area.
- Remove the new-bus attachment from npx. This was only ever useful for
external co-processors using IRQ13, but those have not been supported
for several years. npxinit() is now called much earlier during boot
(init386()) similar to amd64.
- Implement PT_{GET,SET}XSTATE and I386_GET_XFPUSTATE.
- npxsave() is now only called from context switch contexts so it can
use XSAVEOPT.
Differential Revision: https://reviews.freebsd.org/D1058
Reviewed by: kib
Tested on: FreeBSD/i386 VM under bhyve on Intel i5-2520
824fc46089