target/ppc: Split out gen_load_locked

Leave only the minimal amount of code within the LDAR macro,
moving the rest of the code into gen_load_locked.  Use MO_ALIGN
and remove the explicit call to gen_check_align.

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