target-arm: Factor out handling of SRS instruction

target-arm: Factor out handling of SRS instruction

Factor out the handling of the SRS instruction rather than
duplicating it between the Thumb and ARM decoders. This in
passing fixes two bugs in the Thumb decoder's SRS handling
which didn't exist in the ARM decoder:
 * (LP:1079080) storing CPSR rather than SPSR (fixed in the
   ARM decoder in commit c67b6b71 in 2009)
 * failing to free the 'addr' TCG temp in the writeback case

Reported-by: Cesson Vincent <vcesson@stmi.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>