target/arm: Diagnose base == pc for LDM/STM

We have been using store_reg and not store_reg_for_load when writing
back a loaded value into the base register.  At first glance this is
incorrect when base == pc, however that case is UNPREDICTABLE.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190904193059.26202-32-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>