Reorganize memchr selection.

Reorganize memchr selection.

The patch cleans up the auto configury mechanism used to select
different implementations of memchr for various architecture versions.

The approach here is to remove the selection of memchr within automake
and instead use complimentary logic in memchr-stub.c and memchr.S to
choose between the gerneric memchr.c implementation or one of the
architecture specific implementations.

This patch also changes the selection criteria inline with the
previous proposal here:

https://sourceware.org/ml/newlib/2015/msg00752.html

but using the ACLE predefines.

Regressed for armv7-a armv5 armv8-a, correct selection of memcpy
implementation by manual inspection of a test program built for these
three architectures.