libc types exposed by libstd

[arcnmx]

The libc merge was a breaking change: c_char on ARM Linux was i8 but is now u8. This is a bugfix, which is fair, it was wrong before. This type is exposed via CStr::as_ptr() however, and there was no obvious way to refer to it, so users would depend on and use the crates.io libc::c_char, which only corresponds if using (old rustc + libc 0.1) or (new rustc + libc 0.2).

Note that os::raw::c_char is still wrong/old, so even using that would break.

cc @alexcrichton