target/ppc: Implement the rest of gen_st_atomic

The store twin case was stubbed out.  For now, implement it only within
a serial context, forcing parallel execution to synchronize.  It would
be possible to implement with a cmpxchg loop, if we care, but the loose
alignment requirements (simply no crossing 32-byte boundary) might send
us back to the serial context anyway.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>