cleanup cpu-exec.c, part 0/N: consolidate handle_cpu_signal

handle_cpu_signal is very nearly copy-paste code for each target, with a
few minor variations.  This patch sets up appropriate defaults for a
generic handle_cpu_signal and provides overrides for particular targets
that did things differently.  Fixing things like the persistent (XXX:
use sigsetjmp) should now become somewhat easier.

Previous comments on this patch suggest that the "activate soft MMU for
this block" comments refer to defunct functionality.  I have removed
such blocks for the appropriate targets in this patch.

Signed-off-by: Nathan Froyd <froydnj@codesourcery.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
