x86: Stop CPUID from claiming we support xsave.

x86: Stop CPUID from claiming we support xsave.

xsave is a fairly complex feature which we don't support in gem5, but
we do report that we support it through CPUID. It looks like I confused
it with FXSAVE which is an instruction related to SSE. This change
turns that bit back off again.

Change-Id: I00fc79168c5f7095b5241e870a4c8782e4385425
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20169
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Pouya Fotouhi <pfotouhi@ucdavis.edu>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>