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/arm/add.S

@@ -9,4 +9,5 @@ main:
     beq 1f
     bl lkmc_assert_fail
 1:
+    mov r0, #0
     bx lr

baremetal/arch/arm/multicore.S

@@ -32,6 +32,7 @@ spinlock_start:
     wfe
     cmp r0, #0
     beq spinlock_start
+    mov r0, #0
     bx lr
 spinlock:
     .skip 4

baremetal/arch/arm/regs.S

@@ -5,4 +5,5 @@ main:
     /* test-gdb-r0 */
     mov r1, #2
     /* test-gdb-r1 */
+    mov r0, #0
     bx lr