baremetal: allow arbitrary exit status with the magic string

test-baremetal: fix missing setting x0 return value

Examples were just returning on ret without setting x0, which led to
failures... those were not noticed because of how broken the testing system
was ;-)

baremetal/lib/syscalls.c

@@ -62,13 +62,23 @@ int _write(int file, char *ptr, int len) {
     return len;
 }
 
-/* Only 0 is supported for now, arm semihosting cannot handle other values. */
 void _exit(int status) {
+    if (status != 0) {
+        /* https://github.com/cirosantilli/linux-kernel-module-cheat#magic-failure-string */
+        printf("lkmc_exit_status_%d\n", status);
+    }
 #if defined(GEM5)
     LKMC_M5OPS_EXIT;
 #else
 #if defined(__arm__)
-    __asm__ __volatile__ ("mov r0, #0x18; ldr r1, =#0x20026; svc 0x00123456" : : : "r0", "r1");
+    __asm__ __volatile__ (
+        "mov r0, #0x18\n"
+        "ldr r1, =#0x20026\n"
+        "svc 0x00123456\n"
+        :
+        :
+        : "r0", "r1"
+    );
 #elif defined(__aarch64__)
     /* TODO actually use the exit value here, just for fun. */
     __asm__ __volatile__ (