compatible method of handling floating point exceptions.
Make kernel support for teh fpu non-optional (486SX should still work).
Only 386 cpus support external fpu, and i386 support was removed years ago.
This means that the npx code no longer uses port 0xf0 or interupt 13.
All the "npx at isa" lines go from the configs, arch/i386/isa/npx.c
is now mandatory for all i386 kernels.
I've renamed npxinit() to fpuinit() and npxinit_cpu() to fpuinit_cpu()
to match the very similar amd64 functions.
The fpu of the boot cpu is now initialised by a direct call from
cpu_configure(), this enables FP emulation for a 486SX.
(for amd64 the cr0 values are set in locore.S and similar).
This fixes a long-standing bug in linux_setregs() - which did not
save the fpu regsiters if they were active.
I've test booted a single cpu i386 kernel (using anita).
amd64 builds - none of teh changes should affect it.
The i386 XEN kernels build, but I'm not sure where they set cr0, and
it might have got lost!
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