Fix x86 strncat optimized implementation for large sizes

Similar to BZ#19387, BZ#21014, and BZ#20971, both x86 sse2 strncat
optimized assembly implementations do not handle the size overflow
correctly.

The x86_64 one is in fact an issue with strcpy-sse2-unaligned, but
that is triggered also with strncat optimized implementation.

This patch uses a similar strategy used on 3daef2c8, where
saturared math is used for overflow case.

Checked on x86_64-linux-gnu and i686-linux-gnu.  It fixes BZ #19390.

	[BZ #19390]
	* string/test-strncat.c (test_main): Add tests with SIZE_MAX as
	maximum string size.
	* sysdeps/i386/i686/multiarch/strcat-sse2.S (STRCAT): Avoid overflow
	in pointer addition.
	* sysdeps/x86_64/multiarch/strcpy-sse2-unaligned.S (STRCPY):
	Likewise.