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/LibPthread/pthread.h

@@ -146,6 +146,4 @@ int pthread_rwlockattr_setpshared(pthread_rwlockattr_t*, int);
 
 int pthread_atfork(void (*prepare)(void), void (*parent)(void), void (*child)(void));
 
-void __pthread_key_destroy_for_current_thread();
-
 __END_DECLS