Kernel+LibPthread: Implement pthread_join()

It's now possible to block until another thread in the same process has
exited. We can also retrieve its exit value, which is whatever value it
passed to pthread_exit(). :^)

Userland/tt.cpp

@@ -0,0 +1,25 @@
+#include <pthread.h>
+#include <stdio.h>
+
+int main(int, char**)
+{
+    printf("Hello from the first thread!\n");
+    pthread_t thread_id;
+    int rc = pthread_create(&thread_id, nullptr, [](void*) -> void* {
+        printf("Hi there, from the second thread!\n");
+        pthread_exit((void*)0xDEADBEEF);
+        return nullptr;
+    }, nullptr);
+    if (rc < 0) {
+        perror("pthread_create");
+        return 1;
+    }
+    void* retval;
+    rc = pthread_join(thread_id, &retval);
+    if (rc < 0) {
+        perror("pthread_join");
+        return 1;
+    }
+    printf("Okay, joined and got retval=%p\n", retval);
+    return 0;
+}