target-ppc: retain l{w,d}arx loaded value

We do this so we can check on the corresponding stc{w,d}x. whether the
value has changed.  It's a poor man's form of implementing atomic
operations and is valid only for NPTL usermode Linux emulation.

Signed-off-by: Nathan Froyd <froydnj@codesourcery.com>
Signed-off-by: malc <av1474@comtv.ru>
