Add a new randomized memcpy test for copies up to 256 bytes. The distribution

Add a new randomized memcpy test for copies up to 256 bytes.  The distribution
of the size and alignment is based on a trace of SPEC2006.  Instead of
repeating the same copy over and over again like the existing tests, it times
several thousand different copies to more accurately estimate the overhead of
branch prediction.

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