Fix powerpc64/power7 memchr for large input sizes

Current optimized powercp64/power7 memchr uses a strategy to check for
p versus align(p+n) (where 'p' is the input char pointer and n the
maximum size to check for the byte) without taking care for possible
overflow on the pointer addition in case of large 'n'.

It was triggered by 3038145c where default rawmemchr (used to
created ppc64 rawmemchr in ifunc selection) now uses memchr (p, c, (size_t)-1)
on its implementation.

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

Checked on powerpc64le-linux-gnu.

	[BZ# 20971]
	* sysdeps/powerpc/powerpc64/power7/memchr.S (__memchr): Avoid
	overflow in pointer addition.
	* string/test-memchr.c (do_test): Add an argument to pass as
	the size on memchr.
	(test_main): Add check for SIZE_MAX.