target-i386: Pass X86CPU to cpu_x86_set_a20()

Prepares for cpu_interrupt() changing argument to CPUState.

While touching it, rename to x86_cpu_...() now that it takes an X86CPU.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>