define sync_core for x86 PIC

Pushing/popping the reserved ebx register is surely less expensive
than a memory barrier.

Note that since ebx is a callee-save register, this is even safe for
signals (i.e. it would be safe even if we needed the value that cpuid
puts in %%ebx).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>