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_ppc.h

@@ -1759,7 +1759,7 @@ static void asm_stack_check(ASMState *as, BCReg topslot,
   emit_tai(as, PPCI_LWZ, tmp, tmp, offsetof(lua_State, maxstack));
   if (pbase == RID_TMP)
     emit_getgl(as, RID_TMP, jit_base);
-  emit_getgl(as, tmp, jit_L);
+  emit_getgl(as, tmp, cur_L);
   if (allow == RSET_EMPTY)  /* Spill temp. register. */
     emit_tai(as, PPCI_STW, tmp, RID_SP, SPOFS_TMPW);
 }