target-i386: Fix SSE status flag corruption

When we restore the mxcsr register with FXRSTOR, or set it with gdb,
we need to update the various SSE status flags in CPUX86State

Reported-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>
