target/arm: Correct load exclusive pair atomicity

target/arm: Correct load exclusive pair atomicity

We are not providing the required single-copy atomic semantics for
the 64-bit operation that is the 32-bit paired load.

At the same time, leave the entire 64-bit value in cpu_exclusive_val
and stop writing to cpu_exclusive_high.  This means that we do not
have to re-assemble the 64-bit quantity when it comes time to store.

At the same time, drop a redundant temporary and perform all loads
directly into the cpu_exclusive_* globals.

Tested-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20170815145714.17635-3-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>