x86/fxsave: bring in line with recent xsave adjustments

x86/fxsave: bring in line with recent xsave adjustments

Defer the FIP/FDP pointer reset needed on AMD CPUs to the restore path,
and switch from using EMMS to FFREE here too (to be resistant against
eventual future CPUs without MMX support). Also switch from using an
almost typeless pointer in fpu_fxrstor() to a properly typed one, thus
telling the compiler the truth about which memory gets accessed.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>