Add pthread_sigqueue.

pthread_sigqueue is a gnu libc extension, however I have a use case for it,
hence why I'm adding it.

libc-test/build.rs

@@ -3270,6 +3270,12 @@ fn test_linux(target: &str) {
             // Needs musl 1.2.3 or later.
             "pthread_getname_np" if musl => true,
 
+            // pthread_sigqueue uses sigval, which was initially declared
+            // as a struct but should be defined as a union. However due
+            // to the issues described here: https://github.com/rust-lang/libc/issues/2816
+            // it can't be changed from struct.
+            "pthread_sigqueue" => true,
+
             _ => false,
         }
     });

src/unix/linux_like/mod.rs

@@ -1729,6 +1729,7 @@ extern "C" {
         val: *mut ::c_int,
     ) -> ::c_int;
     pub fn pthread_rwlockattr_setpshared(attr: *mut pthread_rwlockattr_t, val: ::c_int) -> ::c_int;
+    pub fn pthread_sigqueue(thread: ::pthread_t, sig: ::c_int, value: ::sigval) -> ::c_int;
     pub fn ptsname_r(fd: ::c_int, buf: *mut ::c_char, buflen: ::size_t) -> ::c_int;
     pub fn clearenv() -> ::c_int;
     pub fn waitid(idtype: idtype_t, id: id_t, infop: *mut ::siginfo_t, options: ::c_int)