ARM64: Add support for ARM64e pointer authentication codes (PAC).

Contributed by Peter Cawley. #559

src/lj_jit.h

@@ -273,6 +273,9 @@ typedef struct GCtrace {
   BCIns startins;	/* Original bytecode of starting instruction. */
   MSize szmcode;	/* Size of machine code. */
   MCode *mcode;		/* Start of machine code. */
+#if LJ_ABI_PAUTH
+  ASMFunction mcauth;	/* Start of machine code, with ptr auth applied. */
+#endif
   MSize mcloop;		/* Offset of loop start in machine code. */
   uint16_t nchild;	/* Number of child traces (root trace only). */
   uint16_t spadjust;	/* Stack pointer adjustment (offset in bytes). */