x86: disable SSE on 32-bit hosts

On 64-bit hosts we are disabling SSE and SSE2.  Depending on the
compiler however it may use movq instructions for 64-bit transfers
even when targeting 32-bit processors; when CR4.OSFXSR is not set,
this results in an undefined opcode exception, so tell the compiler
to avoid those instructions on 32-bit hosts as well.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>