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

[Helge Deller]

Words are stored in big endian in the guest memory 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.

The bug can be verified with qemu >= v7.2 on any little-endian host,
when starting the armeb binary of the upx program, which just hangs
without this patch.

Signed-off-by: Helge Deller <del...@gmx.de>
Cc: Richard Henderson <richard.hender...@linaro.org>
Cc: Peter Maydell <peter.mayd...@linaro.org>
Cc: qemu-sta...@nongnu.org
Reported-by: "Markus F.X.J. Oberhumer" <mar...@oberhumer.com>
Reported-by: John Reiser <jrei...@bitwagon.com>
Closes: https://github.com/upx/upx/issues/687

diff --git a/linux-user/arm/cpu_loop.c b/linux-user/arm/cpu_loop.c
index a992423257..ff0bff7c63 100644
--- a/linux-user/arm/cpu_loop.c
+++ b/linux-user/arm/cpu_loop.c
@@ -117,8 +117,8 @@ static void arm_kernel_cmpxchg32_helper(CPUARMState *env)
 {
     uint32_t oldval, newval, val, addr, cpsr, *host_addr;

-    oldval = env->regs[0];
-    newval = env->regs[1];
+    oldval = tswap32(env->regs[0]);
+    newval = tswap32(env->regs[1]);
     addr = env->regs[2];

     mmap_lock();