Use generic memset/memcpy/memmove in benchtests

Use the generic C memset/memcpy/memmove in benchtests since comparing
against a slow byte-oriented implementation makes no sense.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>

2019-08-29  Wilco Dijkstra  <wdijkstr@arm.com>

	* benchtests/bench-memcpy.c (simple_memcpy): Remove.
	(generic_memcpy): Include generic C memcpy.
	* benchtests/bench-memmove.c (simple_memmove): Remove.
	(generic_memmove): Include generic C memmove.
	* benchtests/bench-memset.c (simple_memset): Remove.
	(generic_memset): Include generic C memset.
	* benchtests/bench-memset-large.c (simple_memset): Remove.
	(generic_memset): Include generic C memset.
	* benchtests/bench-memset-walk.c (simple_memset): Remove.
	(generic_memset): Include generic C memset.
	* string/memcpy.c (MEMCPY): Add defines to enable redirection.
	* string/memset.c (MEMSET): Likewise.
	* sysdeps/x86_64/memcopy.h: Remove empty file.