Reorganize memcpy selection.

Reorganize memcpy selection.

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

The approach here is to remove the selection of memcpy within automake
and instead use complimentary logic in memcpy-stub.c and memcpy.S to
choose between the generic memcpy.c implemenation or one of the
architecture specific memcpy*.S implemenations.

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

This revised patch flips the remaining preprocessor logic in
memcpy-stub.c to use ACLE defines as requested in the previous review
and removes the now disused HAVE_ARMV7A and HAVE_ARMV8A configure.in
support.