Added SAXPY test to benchmark

I thought I'd see if I can get a lower throughput than any of the existing tests by doing an even simpler calculation than the existing vector arithmetic benchmark.  Instead, the GFLOPs count for saxpy turned out to be roughly 10% *higher*.  I guess alpha adds to the flops count but not to the bandwidth or cache use.

Not sure if this is worth merging, but I figure no harm in letting you decide.