linux-user/arm: Fix return value of SYS_cacheflush

Although the emulated cacheflush syscall does nothing, it still needs to
return zero to indicate success.

Cc: qemu-stable@nongnu.org
Signed-off-by: J. Neuschäfer <j.neuschaefer@gmx.net>
Message-id: 20250613-cache-v1-1-ee9f4a9ba81b@gmx.net
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
(cherry picked from commit 5ad2b1f443)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

linux-user/arm/cpu_loop.c

@@ -395,6 +395,7 @@ void cpu_loop(CPUARMState *env)
                     switch (n) {
                     case ARM_NR_cacheflush:
                         /* nop */
+                        env->regs[0] = 0;
                         break;
                     case ARM_NR_set_tls:
                         cpu_set_tls(env, env->regs[0]);