LibPthread: Implement a basic first pthread mutex

This patch adds these API's:

- pthread_mutex_init()
- pthread_mutex_lock()
- pthread_mutex_unlock()

No mutex attributes are supported yet, so we only do the simplest mutex
wihout recursive locking.

Userland/tt.cpp

@@ -1,15 +1,24 @@
 #include <pthread.h>
 #include <stdio.h>
+#include <unistd.h>
+#include <stdlib.h>
 
-int main(int, char**)
+static int mutex_test();
+
+int main(int argc, char** argv)
 {
+    if (argc == 2 && *argv[1] == 'm')
+        return mutex_test();
+
     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);
+    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;
@@ -23,3 +32,41 @@ int main(int, char**)
     printf("Okay, joined and got retval=%p\n", retval);
     return 0;
 }
+
+static pthread_mutex_t mutex;
+
+int mutex_test()
+{
+    int rc = pthread_mutex_init(&mutex, nullptr);
+    if (rc < 0) {
+        perror("pthread_mutex_init");
+        return 1;
+    }
+    pthread_t thread_id;
+    rc = pthread_create(
+        &thread_id, nullptr, [](void*) -> void* {
+            printf("I'm the secondary thread :^)\n");
+            for (;;) {
+                pthread_mutex_lock(&mutex);
+                printf("Second thread stole mutex\n");
+                sleep(1);
+                printf("Second thread giving back mutex\n");
+                pthread_mutex_unlock(&mutex);
+                sleep(1);
+            }
+            pthread_exit((void*)0xDEADBEEF);
+            return nullptr;
+        },
+        nullptr);
+    if (rc < 0) {
+        perror("pthread_create");
+        return 1;
+    }
+    for (;;) {
+        pthread_mutex_lock(&mutex);
+        printf("Obnoxious spam!\n");
+        pthread_mutex_unlock(&mutex);
+        usleep(10000);
+    }
+    return 0;
+}