Kernel/LibC: Allocate thread stacks in userspace

This adds support for pthread function for stack manipulation.

Also keep track of main threads stack in the uthread

userspace/libraries/LibC/include/bits/types/pthread_attr_t.h

@@ -16,6 +16,7 @@ typedef struct
 	int schedpolicy;
 	int detachstate;
 	int scope;
+	void* stackaddr;
 	size_t stacksize;
 	size_t guardsize;
 } pthread_attr_t;

userspace/libraries/LibC/include/bits/types/uthread.h

@@ -49,6 +49,7 @@ struct uthread
 	pid_t id;
 	pthread_attr_t attr;
 	int errno_;
+	int libc_owns_stack;
 	int cancel_type;
 	int cancel_state;
 	volatile int canceled;