reorganize thread exit code, make pthread_exit call cancellation handlers

src/thread/pthread_exit.c

-#include "pthread_impl.h"
-
-#undef pthread_self
-
-void pthread_exit(void *result)
-{
-	int i;
-	struct pthread *self = pthread_self();
-	self->result = result;
-
-	a_dec(&libc.threads_minus_1);
-	if (libc.threads_minus_1 < 0)
-		exit(0);
-
-	LOCK(&self->exitlock);
-
-	if (self->tsd_used) for (i=0; i<PTHREAD_KEYS_MAX; i++)
-		if (self->tsd[i] && libc.tsd_keys[i])
-			libc.tsd_keys[i](self->tsd[i]);
-
-	if (self->detached && self->map_base)
-		__unmapself(self->map_base, self->map_size);
-
-	__syscall_exit(0);
-}