ia64: Fix memchr for large input sizes (BZ #22603)

Current optimized ia64 memchr uses a strategy to check for last address
by adding the input one with expected size.  However it does not take
care for possible overflow.

It was triggered by 3038145c where default rawmemchr now uses memchr
(p, c, (size_t)-1).

This patch fixes it by implement a satured addition where overflows
sets the maximum pointer size to UINTPTR_MAX.

Checked on ia64-linux-gnu where it fixes both stratcliff and
test-rawmemchr failures.

	Adhemerval Zanella  <adhemerval.zanella@linaro.org>
	James Clarke <jrtc27@jrtc27.com>

	[BZ #22603]
	* sysdeps/ia64/memchr.S (__memchr): Avoid overflow in pointer
	addition.

(cherry picked from commit 3bb1ef58)