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_obj.h

@@ -536,7 +536,7 @@ typedef struct global_State {
   lua_CFunction panic;	/* Called as a last resort for errors. */
   BCIns bc_cfunc_int;	/* Bytecode for internal C function calls. */
   BCIns bc_cfunc_ext;	/* Bytecode for external C function calls. */
-  GCRef jit_L;		/* Current JIT code lua_State. */
+  GCRef cur_L;		/* Currently executing lua_State. */
   MRef jit_base;	/* Current JIT code L->base or NULL. */
   MRef ctype_state;	/* Pointer to C type state. */
   GCRef gcroot[GCROOT_MAX];  /* GC roots. */