Fix the Solaris pthread_t raw type in std to match what's in libc

The old type causes failures when building cargo 0.20.0 after
changeset 8304e06b5 in the libc repo.

src/libstd/os/solaris/raw.rs

@@ -32,7 +32,7 @@ use os::unix::raw::{uid_t, gid_t};
 #[stable(feature = "raw_ext", since = "1.1.0")] pub type time_t = i64;
 
 #[stable(feature = "pthread_t", since = "1.8.0")]
-pub type pthread_t = usize;
+pub type pthread_t = u32;
 
 #[repr(C)]
 #[derive(Clone)]