LibC+LibPthread: Implement pthread_atfork()

This required a bit of rearchitecture, as pthread_atfork() required a
mutex, and duplicating a mutex impl for it was silly.
As such, this patch moves some standalone bits of pthread into LibC and
uses those to implement atfork().
It should be noted that for programs that don't use atfork(), this
mechanism only costs two atomic loads (as opposed to the normal mutex
lock+unlock) :^)

Userland/Libraries/LibC/CMakeLists.txt

@@ -17,6 +17,7 @@ set(LIBC_SOURCES
     mntent.cpp
     netdb.cpp
     poll.cpp
+    pthread_integration.cpp
     pwd.cpp
     qsort.cpp
     scanf.cpp