Move 64-bit-specific NetBSD stuff around

src/unix/bsd/netbsdlike/netbsd/mod.rs

-pub type c_long = i64;
-pub type c_ulong = u64;
 pub type clock_t = ::c_uint;
 pub type suseconds_t = ::c_int;
 pub type dev_t = u64;
@@ -615,3 +613,6 @@ extern {
                      locale: *const ::c_char,
                      base: ::locale_t) -> ::locale_t;
 }
+
+mod other;
+pub use self::other::*;

src/unix/bsd/netbsdlike/netbsd/other/b64/mod.rs

+pub type c_long = i64;
+pub type c_ulong = u64;

src/unix/bsd/netbsdlike/netbsd/other/mod.rs

+cfg_if! {
+    if #[cfg(target_arch = "x86_64")] {
+        mod b64;
+        pub use self::b64::*;
+    } else {
+        // Unknown target_arch
+    }
+}