rustc: Set the exceeding_bitshifts lint to Allow

There's currently a bug in it which fires erroneously on cross compiles,
preventing new nightlies from being generated. This can be reset back to Deny
once it's been fixed.

cc #18587
auto merge of #18589 : alexcrichton/rust/issue-18587-warn, r=jakub-

There's currently a bug in it which fires erroneously on cross compiles,
preventing new nightlies from being generated. This can be reset back to Deny
once it's been fixed.

cc #18587