MFamd64: Add support for extended FPU states on i386. This includes
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