support @repr u8 on enums in C backend

[Gankra]

In rust #[repr(u8)] says a C-like enum's underlying type should be a u8. #[repr(C)] says it should have underlying type int (or whatever matches the dominant C ABI of the target platform).

In C++ this attribute could be lowered to an enum class with the following syntax:

enum class MyEnum: uint8_t { ... }

In C I'm not sure what the equivalent is -- possibly in the most extreme generating #defines. Although i vaguely recall hearing newer C's getting an equivalent of the C++ syntax here?