Compute sum of squared residuals faster in gels!

This was taking a substantial portion of the function runtime. Before:

```julia
julia> srand(1776); @time Base.LinAlg.LAPACK.gels!('N', rand(1000, 100), rand(1000, 100000));
elapsed time: 3.392343747 seconds (2374561640 bytes allocated)
```

After:

```julia
julia> srand(1776); @time Base.LinAlg.LAPACK.gels!('N', rand(1000, 100), rand(1000, 100000));
elapsed time: 2.074082745 seconds (910542724 bytes allocated)
```

The other work `gels!` performs on the outputs takes something like 10% of the function runtime in this test case, which is acceptable, although I was a little surprised the wrapper does anything more than just call the LAPACK function.