target-arm: Fix handling of STM (user) with r15 in register list

The A32 encoding of LDM distinguishes LDM (user) from LDM (exception
return) based on whether r15 is in the register list. However for
STM (user) there is no equivalent distinction. We were incorrectly
treating "r15 in list" as indicating exception return for both LDM
and STM, with the result that an STM (user) involving r15 went into
an infinite loop. Fix this; note that the value stored for r15
in this case is the current PC regardless of our current mode.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1426015125-5521-1-git-send-email-peter.maydell@linaro.org