target/arm: Abstract out pbit/wbit tests in ARM ldr/str decode

In the ARM ldr/str decode path, rather than directly testing
"insn & (1 << 21)" and "insn & (1 << 24)", abstract these
bits out into wbit and pbit local flags. (We will want to
do more tests against them to determine whether we need to
provide syndrome information.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>