x64/LJ_GC64: Add missing backend support and enable JIT compilation.

Contributed by Peter Cawley.

src/vm_x64.dasc

@@ -2401,8 +2401,7 @@ static void build_subroutines(BuildCtx *ctx)
   |  movzx RCd, byte [rbp-8]		// Reconstruct exit number.
   |  mov RCH, byte [rbp-16]
   |  mov [rbp-8], r15; mov [rbp-16], r14
-  |  // Caveat: DISPATCH is rbx.
-  |  mov DISPATCH, [ebp]
+  |  // DISPATCH is preserved on-trace in LJ_GC64 mode.
   |  mov RAd, [DISPATCH+DISPATCH_GL(vmstate)]	// Get trace number.
   |  set_vmstate EXIT
   |  mov [DISPATCH+DISPATCH_J(exitno)], RCd