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

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

baremetal/lib/syscalls_asm.S

@@ -0,0 +1,35 @@
+#include <lkmc.h>
+
+/* This is implemented in assembly so that it does not use the stack,
+ * and thus can be called safely from programs without the bootloader.
+ * C signature:
+ *
+ * void _exit(int status)
+ *
+ * If only there was a GCC attribute to create such a function!
+ */
+.text
+.global _exit
+_exit:
+#if LKMC_GEM5
+    LKMC_M5OPS_EXIT_ASM
+#else
+/* Use semihosting:
+ * https://github.com/cirosantilli/linux-kernel-module-cheat#semihosting */
+#if defined(__arm__)
+    mov r0, #0x18
+    ldr r1, =#0x20026
+    svc 0x00123456
+#elif defined(__aarch64__)
+    mov x1, 0x26
+    movk x1, 2, lsl 16
+    ldr x2, =.Lsemihost_args
+    str x1, [x2, 0]
+    str x0, [x2, 8]
+    mov x1, x2
+    mov w0, 0x18
+    hlt 0xf000
+.Lsemihost_args:
+    .skip 16
+#endif
+#endif