Replace b = a*a with A_mul_B(b, a, a).

As discussed on julia-dev, the prior version is largely a GC test, since it
allocates memory for the result on each iteration. This version more directly
tests the BLAS library.
Merge pull request #4045 from blakejohnson/matmultest

Replace b = a*a with A_mul_B(b, a, a) in BLAS level3 perf test.