Remove the `libc` version of `core::ffi` primitives

[tgross35]

Now that these exist in core, there isn't any reason to duplicate the definitions here. For 1.0 we should remove the types from libc in favor of using core::ffi.

Ideally we would also make a change in 0.2 to reexport the types rather than redefining them. We can't do this yet because our MSRV is 1.63 and core::ffi::c_char is not available until 1.64. However even once the MSRV upgrade happens, we need to be a bit careful; Rust 1.85 (rust-lang/rust#132975) and libc 0.2.169 corrected the signedness of c_char on a number of platforms which was accepted minor breakage. If we switch to reexporting then we break things a second time for new libc with older Rust.