Save currently executing lua_State in g->cur_L.

This is only a good approximation due to deficiencies in the design of
the Lua/C API. It indicates _some_ valid state that is/was executing.
Also reorder L->cframe stores to achieve a synchronously consistent state.

src/lj_asm.c

@@ -342,7 +342,7 @@ static Reg ra_rematk(ASMState *as, IRRef ref)
     emit_getgl(as, r, jit_base);
   } else if (emit_canremat(ASMREF_L) && ir->o == IR_KPRI) {
     lua_assert(irt_isnil(ir->t));  /* REF_NIL stores ASMREF_L register. */
-    emit_getgl(as, r, jit_L);
+    emit_getgl(as, r, cur_L);
 #if LJ_64
   } else if (ir->o == IR_KINT64) {
     emit_loadu64(as, r, ir_kint64(ir)->u64);