LibC+LibPthread: Implement function forwarding for libpthread

GCC will insert various calls to pthread functions when compiling
C++ code with static initializers, even when the user doesn't link
their program against libpthread explicitly.

This is used to make static initializers thread-safe, e.g. when
building a library that does not itself use thread functionality
and thus does not link against libpthread - but is intended to
be used with other code that does use libpthread explicitly.

This makes these symbols available in libc.

Userland/Libraries/LibC/CMakeLists.txt

@@ -20,7 +20,9 @@ set(LIBC_SOURCES
     mntent.cpp
     netdb.cpp
     poll.cpp
+    pthread_forward.cpp
     pthread_integration.cpp
+    pthread_tls.cpp
     pwd.cpp
     qsort.cpp
     scanf.cpp