c-stack: stop worrying about stack direction

c-stack: stop worrying about stack direction

* lib/c-stack.c (find_stack_direction): Remove.
(segv_handler): Don't worry about stack direction growth, as it's
too much of a pain to configure this correctly, given how compilers
are optimizing-away our stack-growth detection code.  Instead, assume
that any access to just before or just after the stack is OK.
* m4/c-stack.m4 (AC_SYS_XSI_STACK_OVERFLOW_HEURISTIC):
Don't require AC_FUNC_ALLOCA; no longer needed.