target/ppc: Implement the rest of gen_ld_atomic

These cases were stubbed out.  For now, implement them only within
a serial context, forcing parallel execution to synchronize.  It
would be possible to implement these with cmpxchg loops, if we care.

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