target/arm: Correct exclusive store cmpxchg memop mask

When we perform the atomic_cmpxchg operation we want to perform the
operation on a pair of 32-bit registers. Previously we were just passing
the register size in which was set to MO_32. This would result in the
high register to be ignored. To fix this issue we hardcode the size to
be 64-bits long when operating on 32-bit pairs.

Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Tested-by: Portia Stephens <portia.stephens@xilinx.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20170815145714.17635-2-richard.henderson@linaro.org
Message-Id: <bc18dddca56e8c2ea4a3def48d33ceb5d21d1fff.1502488636.git.alistair.francis@xilinx.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>