target/arm: Return correct result for LDG when ATA=0

target/arm: Return correct result for LDG when ATA=0

The LDG instruction loads the tag from a memory address (identified
by [Xn + offset]), and then merges that tag into the destination
register Xt. We implemented this correctly for the case when
allocation tags are enabled, but didn't get it right when ATA=0:
instead of merging the tag bits into Xt, we merged them into the
memory address [Xn + offset] and then set Xt to that.

Merge the tag bits into the old Xt value, as they should be.

Cc: qemu-stable@nongnu.org
Fixes: c15294c1e36a7dd9b25 ("target/arm: Implement LDG, STG, ST2G instructions")
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>