linux-user/armeb: Fix __kernel_cmpxchg() for armeb

linux-user/armeb: Fix __kernel_cmpxchg() for armeb

Commit 7f4f0d9ea870 ("linux-user/arm: Implement __kernel_cmpxchg with host
atomics") switched to use qatomic_cmpxchg() to swap a word with the memory
content, but missed to endianess-swap the oldval and newval values when
emulating an armeb CPU, which expects words to be stored in big endian in
the guest memory.

The bug can be verified with qemu >= v7.0 on any little-endian host, when
starting the armeb binary of the upx program, which just hangs without
this patch.

Cc: qemu-stable@nongnu.org
Signed-off-by: Helge Deller <deller@gmx.de>
Reported-by: "Markus F.X.J. Oberhumer" <markus@oberhumer.com>
Reported-by: John Reiser <jreiser@BitWagon.com>
Closes: https://github.com/upx/upx/issues/687
Message-Id: <ZMQVnqY+F+5sTNFd@p100>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>