Deal with suseconds_t

For 32 bit linux with GNU glibc and _TIME_BITS=64,
the generated C tests for the members of timeval will fail if the
struct member tv_usec is of type suseconds_txt:

suseconds_t* __test_field_type_timeval_tv_usec(struct timeval* b) {
   return &b->tv_usec;
}

Where timeval.tv_usec is actually __suseconds64_t (64 bits) while
suseconds_t is still 32 bits.

Author: snogge

src/unix/mod.rs

@@ -66,7 +66,12 @@ s! {
 
     pub struct timeval {
         pub tv_sec: time_t,
+        #[cfg(not(all(target_env = "gnu", target_os = "linux", target_pointer_width = "32")))]
         pub tv_usec: suseconds_t,
+        // For 64 bit time on 32 bit linux glibc, suseconds_t is still
+        // a 32 bit type.  Using it here will break the tests.
+        #[cfg(all(target_env = "gnu", target_os = "linux", target_pointer_width = "32"))]
+        pub tv_usec: i64
     }
 
     // linux x32 compatibility