target/ppc: Use atomic cmpxchg for STQCX

When running in a parallel context, we must use a helper in order
to perform the 128-bit atomic operation.  When running in a serial
context, do the compare before the store.

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