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/crt0_shared.cpp

@@ -31,4 +31,6 @@
 #include <sys/internals.h>
 #include <unistd.h>
 
+#ifndef _DYNAMIC_LOADER
 void* __dso_handle __attribute__((__weak__));
+#endif