target-arm: A32: Use get_mem_index for load/stores

Avoid using IS_USER directly as the MMU-idx to simplify future
changes to the MMU layout.
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NEdgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Message-id: 1400980132-25949-5-git-send-email-edgar.iglesias@gmail.com
Message-id: 1400805738-11889-6-git-send-email-edgar.iglesias@gmail.com
[PMM: parts relating to LDRT/STRT moved into earlier patches]
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>