KVM: selftests: Only muck with SREGS on x86 in mmu_stress_test

Try to get/set SREGS in mmu_stress_test only when running on x86, as the
ioctls are supported only by x86 and PPC, and the latter doesn't yet
support KVM selftests.

Reviewed-by: James Houghton <jthoughton@google.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20241128005547.4077116-7-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>