liblibc: don't use int/uint for intptr_t/uintptr_t

int/uint aren't considered FFI safe, replace them with the actual type they
represent (i64/u64 or i32/u32). This is a breaking change, but at most a cast
to `uint` or `int` needs to be added.

[breaking-change]

src/libnative/io/c_unix.rs

@@ -235,6 +235,7 @@ mod signal {
     pub type sigset_t = u32;
     #[cfg(target_os = "freebsd")]
     #[cfg(target_os = "dragonfly")]
+    #[repr(C)]
     pub struct sigset_t {
         bits: [u32, ..4],
     }