Sparc: fix non-faulting unassigned memory accesses

Sparc: fix non-faulting unassigned memory accesses

Commit b14ef7c9ab41ea824c3ccadb070ad95567cca84e
introduced cpu_unassigned_access() function. On Sparc,
the function does not restore AREG0 used for global CPUState
on function exit, causing bugs with non-faulting unassigned
memory accesses. Alpha, Microblaze and MIPS are not affected.

Fix by restoring AREG0 on exit. Remove excess saving by
do_unassigned_access() functions.

Also ignore unassigned accesses outside of CPU context.

Reported-by: Bob Breuer <breuerr@mc.net>
Tested-by: Bob Breuer <breuerr@mc.net>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>