benchtests: Memory walking benchmark for memset

This benchmark is an attempt to eliminate cache effects from string
benchmarks.  The benchmark walks backward through a large memory area
and sets 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 simple memset benchmark (which is only
really useful for smaller sizes) especially for larger sizes where the
likelihood of the call being done only once is pretty high.

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