target/ppc: Tidy gen_conditional_store

Leave only the minimal amount of code within the STCX macro,
moving the rest of the code into gen_conditional_store.
Remove the explicit call to gen_check_align; the matching LDAX will
have already checked alignment, and we verify the same address.

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