PROT_GROWSDOWN

[Roland McGrath]

> The following change is bogus, _STACK_GROWS_DOWN is defined for i386
> in general, but PROT_GROWS{DOWN,UP} are only defined for the GNU/Linux
> port, not for GNU/Hurd.

Indeed, it should be #ifdef'd if used.  But I can't see the point in the
__stack_prot addition at all.  The code can perfectly well refer to
GL(dl_stack_flags) to see whether to add in PROT_EXEC, and the rest is
compile-time constant anyway.