Kernel+LibPthread+LibC: Create secondary thread stacks in userspace

Have pthread_create() allocate a stack and passing it to the kernel
instead of this work happening in the kernel. The more of this we can
do in userspace, the better.

This patch also unexposes the raw create_thread() and exit_thread()
syscalls since they are now only used by LibPthread anyway.

Libraries/LibC/unistd.cpp

@@ -509,18 +509,6 @@ char* getlogin()
     return nullptr;
 }
 
-int create_thread(void* (*entry)(void*), void* argument)
-{
-    int rc = syscall(SC_create_thread, entry, argument);
-    __RETURN_WITH_ERRNO(rc, rc, -1);
-}
-
-void exit_thread(void* code)
-{
-    syscall(SC_exit_thread, code);
-    ASSERT_NOT_REACHED();
-}
-
 int ftruncate(int fd, off_t length)
 {
     int rc = syscall(SC_ftruncate, fd, length);