benchtests: Memory walking benchmark for memcpy

This benchmark is an attempt to eliminate cache effects from string
benchmarks.  The benchmark walks both ways through a large memory area
and copies different sizes of memory and alignments one at a time
instead of looping around in the same memory area.  This is a good
metric to have alongside the other memcpy benchmarks, especially for
larger sizes where the likelihood of the call being done only once is
pretty high.

	* benchtests/bench-memcpy-walk.c: New file.
	* benchtests/Makefile (string-benchset): Add it.