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/arch/aarch64/no_bootloader/semihost_exit.S

@@ -6,7 +6,7 @@ mystart:
     movk x1, 2, lsl 16
     ldr x2, =semihost_args
     str x1, [x2, 0]
-    mov x0, #0
+    mov x0, 0
     str x0, [x2, 8]
     mov x1, x2
     mov w0, 0x18