std: Don't require bitflags! be u32

If you didn't have a trailing comma at the end of the variants, you could use
any type you wanted, but if you used a trailing comma the macro would
erroneously require the bits be a u32.
auto merge of #17404 : alexcrichton/rust/bitflags-u32, r=sfackler

If you didn't have a trailing comma at the end of the variants, you could use
any type you wanted, but if you used a trailing comma the macro would
erroneously require the bits be a u32.