Fix i686 memchr for large input sizes

Similar to BZ#19387 and BZ#20971, both i686 memchr optimized assembly
implementations (memchr-sse2-bsf and memchr-sse2) do not handle the
size overflow correctly.

It is shown by the new tests added by commit 3daef2c8, where
both implementation fails with size as SIZE_MAX.

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

Checked on i686-linux-gnu.

	[BZ #21014]
	* sysdeps/i386/i686/multiarch/memchr-sse2-bsf.S (MEMCHR): Avoid overflow
	in pointer addition.
	* sysdeps/i386/i686/multiarch/memchr-sse2.S (MEMCHR): Likewise.