ARM64: Add build infrastructure and initial port of interpreter.

src/lib_jit.c

@@ -695,6 +695,8 @@ static uint32_t jit_cpudetect(lua_State *L)
 	   ver >= 60 ? JIT_F_ARMV6_ : 0;
   flags |= LJ_ARCH_HASFPU == 0 ? 0 : ver >= 70 ? JIT_F_VFPV3 : JIT_F_VFPV2;
 #endif
+#elif LJ_TARGET_ARM64
+  /* No optional CPU features to detect (for now). */
 #elif LJ_TARGET_PPC
 #if LJ_HASJIT
 #if LJ_ARCH_SQRT