target-i386: Make x86 mfence and lfence illegal without SSE2

While trying to use qemu -cpu pentium3 to test for incorrect uses of certain
SSE2 instructions, I found that QEMU allowed the mfence and lfence
instructions to be executed even though Pentium 3 doesn't support them.

According to the processor specs (and experience on a real Pentium 3), these
instructions are only available with SSE2, but QEMU is checking for SSE.  The
check for the related sfence instruction is correct (it works with SSE).

This trival patch fixes the test.

Signed-off-by: Martin Simmons <martin@lispworks.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
