kernel: Don't store FPU status into the reserved area of fxsave

kernel: Don't store FPU status into the reserved area of fxsave

Since each time before the old sv_ex_sw is accessed, fnstsw() is
always called, it no longer makes any sense to save the temporary
in the reserved area for fxsave.  And more important: saving the
FPU status into fxsave area overwrites saved xmm register value.