Maintain end of specpdl instead of size

Maintain end of specpdl instead of size

Keep track of the end of specpdl explicitly since that is what we are
comparing against on critical code paths.

* src/eval.c (init_eval_once_for_pdumper, signal_or_quit)
(grow_specpdl_allocation):
* src/fileio.c (Fdo_auto_save):
* src/lisp.h (grow_specpdl):
* src/thread.c (run_thread, Fmake_thread):
* src/thread.h (struct thread_state):
Replace specpdl_size with specpdl_end, according to the equation
specpdl_end = specpdl + specpdl_size.