gem5: fix arm multicore with system.auto_reset_addr = True

baremetal: fix aarch64/no_bootloader/semihost_exit.S which was wrong
because was using unset sp for register block. Tests needed urgently!!

baremetal/arch/arm/multicore.S

@@ -0,0 +1,37 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#arm-multicore */
+
+.global main
+main:
+    mov r0, #0
+    ldr r1, =spinlock
+    str r0, [r1]
+    /* Get CPU ID. */
+    mrc p15, 0, r1, c0, c0, 5
+    ands r1, r1, #3
+    beq cpu0_only
+cpu1_only:
+    mov r0, #1
+    ldr r1, =spinlock
+    str r0, [r1]
+    dmb sy
+    sev
+cpu1_sleep_forever:
+    wfe
+    b cpu1_sleep_forever
+cpu0_only:
+#if !defined(GEM5)
+    /* PCSI CPU_ON. */
+    ldr r0, =0x84000003
+    mov r1, #1
+    ldr r2, =cpu1_only
+    mov r3, #0
+    hvc 0
+#endif
+spinlock_start:
+    ldr r0, spinlock
+    wfe
+    cmp r0, #0
+    beq spinlock_start
+    bx lr
+spinlock:
+    .skip 4