Add jit.tracebarrier() primitive

Create an execution barrier that a JIT trace cannot cross. During
recording, the current trace must end when it reaches a barrier and
then a new root trace starts immediately afterwards.

This is implemented as a simple "nop" C library function. The existing
"trace stitching" mechanism provides the required semantics i.e. stop
current trace, call nop function, start new trace with linkage.

src/lib_jit.c

@@ -99,6 +99,12 @@ LJLIB_CF(jit_status)
   return (int)(L->top - L->base);
 }
 
+/* Calling this forces a trace stitch. */
+LJLIB_CF(jit_tracebarrier)
+{
+  return 0;
+}
+
 LJLIB_PUSH(top-5) LJLIB_SET(os)
 LJLIB_PUSH(top-4) LJLIB_SET(arch)
 LJLIB_PUSH(top-3) LJLIB_SET(version_num)