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/linux/gnu/mod.rs

@@ -1291,6 +1291,7 @@ extern "C" {
         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 mallinfo() -> ::mallinfo;
     pub fn mallinfo2() -> ::mallinfo2;
     pub fn malloc_info(options: ::c_int, stream: *mut ::FILE) -> ::c_int;