target/ppc: Split out gen_st_atomic

Move the guts of ST_ATOMIC to a function.  Use foo_tl for the operations
instead of foo_i32 or foo_i64 specifically.  Use MO_ALIGN instead of an
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>