Clean up canonical mt benchmarks and reorganize extra versions in /mt. All versions support support at least 1/2/4 threads.
- [DH] benchmarks/mt-vvadd/bmark.mk
- [DH] benchmarks/mt-vvadd/dataset.h
- [DH] benchmarks/mt-vvadd/mt-vvadd.c
- [DH] benchmarks/mt-vvadd/vvadd.c