baremetal aarch64: create C version of multicore.S as well

Attempted to do the same for arm, but it failed.

baremetal/arch/aarch64/multicore.c

@@ -0,0 +1,45 @@
+/* https://cirosantilli.com/linux-kernel-module-cheat#arm-multicore
+ *
+ * Beware: things will blow up if the stack for CPU0 grow too much and
+ * reaches that of CPU1. This is why it is so hard to do multithreading
+ * without an OS that manages paging.
+ */
+
+#include <lkmc.h>
+
+uint64_t spinlock = 0;
+
+__asm__(
+".text\n"
+".global lkmc_cpu_not_0\n"
+"lkmc_cpu_not_0:\n"
+/* Put all CPUs except CPU1 to sleep. */
+"    cmp x0, 1\n"
+"    bne .Lsleep_forever\n"
+/* Prepare the stack for CPU1. This is what we need
+ * this assembly function for. */
+"    ldr x0, =(stack_top - 0x1000)\n"
+"    mov sp, x0\n"
+"    bl main_cpu1\n"
+".Lsleep_forever:\n"
+"    wfe\n"
+"    b .Lsleep_forever\n"
+);
+
+static void main_cpu1(void) {
+    spinlock = 1;
+    lkmc_arm_aarch64_dmb(sy);
+    lkmc_arm_aarch64_sev();
+    while (1) {
+        lkmc_arm_aarch64_wfe();
+    }
+}
+
+int main(void) {
+#if !LKMC_GEM5
+    lkmc_aarch64_psci_cpu_on(1, (uint64_t)main_cpu1, 0);
+#endif
+    while (!spinlock) {
+        lkmc_arm_aarch64_wfe();
+    }
+}