mach_o_sys fails to build on platforms where char is unsigned

the [debian build daemons](https://buildd.debian.org/status/package.php?p=rust-mach-o-sys) are failing to build mach_o_sys (`rust-mach-o-sys` is the debian translation of the package name) on platforms which have an unsigned char.

The failures all look like this:

```
error[E0600]: cannot apply unary operator `-` to type `u8`
--> src/swap.rs:581:68
|
581 | pub const BIND_SPECIAL_DYLIB_FLAT_LOOKUP: ::std::os::raw::c_char = -2;
| ^^ cannot apply unary operator `-`
|
= note: unsigned values cannot be negated
```
Discussing this with people on IRC, `infinity0` observed that this looks like much of it is intended to handle a file format. If that's the case, maybe this should be `i8` (or `std::os::raw::c_schar` if you prefer that framing).

I can submit a patch if that would be useful.

Start at src/swap.rs:581 and inspect the other generated constants in that file that use negative values with c_char. Build mach_o_sys on a platform where char is unsigned and verify that the affected constants use a signed representation consistently and the build completes.

Written by the indexing model from the issue text.