rustc: Don't lint about isize/usize in FFI

rustc: Don't lint about isize/usize in FFI

This lint warning was originally intended to help against misuse of the old Rust
`int` and `uint` types in FFI bindings where the Rust `int` was not equal to the
C `int`. This confusion no longer exists (as Rust's types are now `isize` and
`usize`), and as a result the need for this lint has become much less over time.

Additionally, starting with [the RFC for libc][rfc] it's likely that `isize` and
`usize` will be quite common in FFI bindings (e.g. they're the definition of
`size_t` and `ssize_t` on many platforms).

[rfc]: https://github.com/rust-lang/rfcs/pull/1291

This commit disables these lints to instead consider `isize` and `usize` valid
types to have in FFI signatures.