Improve stack-overflow heuristic on GNU/Linux

Improve stack-overflow heuristic on GNU/Linux

Problem reported by Steve Kemp (Bug#27585).
* src/eval.c (near_C_stack_top): Remove.  All uses replaced
by current_thread->stack_top.
(record_in_backtrace): Set current_thread->stack_top.
This is for when the Lisp interpreter calls itself.
* src/lread.c (read1): Set current_thread->stack_top.
This is for recursive s-expression reads.
* src/print.c (print_object): Set current_thread->stack_top.
This is for recursive s-expression printing.
* src/thread.c (mark_one_thread): Get stack top first.
* src/thread.h (struct thread_state.stack_top): Now void *, not char *.