Reinstate a proper computation of the fpu-state size

[Olivier Valentin]

My previous patch erroneously aggregated the fpu-state size computation
for XSAVES and other XSAVE methods. It turns out that the IA32_XSS can
make a difference and that the size has to be read from cpuid(0xD, 0x1)
in one case, and cpuid(0xD, 0x0) in the other.

First patch is a plain revert, to make the new proposal easier to review:
[PATCH gnumach 1/2] Revert "fpu: Fix cpuid feature detection"

The new proposal:
[PATCH gnumach 2/2] fpu: Fix cpuid feature detection

I am happy to squash them toghether if you prefer.

Regards,

Olivier