Fix ia64 executable stack default (bug 22156).

Fix ia64 executable stack default (bug 22156).

As per https://gcc.gnu.org/ml/gcc-patches/2017-09/msg01220.html ia64
defaults to non-executable stacks in the Linux kernel (furthermore,
the use of function descriptors means that trampolines for nested
function pointers never need an executable stack).  glibc however
defines DEFAULT_STACK_PERMS to include PF_X for that architecture,
meaning (a) elf/check-execstack fails and (b) (from code inspection,
not tested, but this is why I think this is a user-visible bug) thread
stacks are unnecessarily mapped with execute permission.  This patch
fixes the DEFAULT_STACK_PERMS definition in question.

Tested (compilation only) with build-many-glibcs.py for ia64.  This
fixes the check-execstack failure.

	[BZ #22156]
	* sysdeps/ia64/stackinfo.h (DEFAULT_STACK_PERMS): Likewise.