Adding basic support of benchmarks into CMake

[dmikushin]

Adding basic support of benchmarks into CMake for single, double, single complex and double complex cases. Each benchmarking target has a suffix to identify the data type, for example ./benchmark_gemm3m_COMPLEX_DOUBLE is a gemm3m.c source compiled with COMPLEX and DOUBLE macros defined.

This PR is done under assumption that the benchmarks folder is not yet wired into CMake compilation.

This PR is not trying to re-create the benchmarks/Makefile behavior. Instead, the purpose of this PR is to compile as many tests as possible from the existing material.

[martin-frbg]

right, the benchmarks are currently not available in the cmake build, but the benchmark suite (which mostly dates back to GotoBLAS) is due for an overhaul anyway.

[dmikushin]

We may still wish to enable it in its current state to provide a lot more code coverage. Otherwise, many functions are not tested at all. For example, cgemm3m/zgemm3m are completely untested, unless we accept these tests for now as a compromise. Of course, someone may work on bringing them to a much better state later.

I'm also using this PR to demonstrate the defect in #4500

[martin-frbg]

The benchmarks make no claim to test for accuracy. The GEMM3M functions are supported on a handful of architectures only, and tests for them are included in test/ctest, though it turns out they are not getting built by default currently.

[dmikushin]

I understand, still having more coverage is better than nothing. I mean, "Perfect is the enemy of good." (c) Voltaire

[martin-frbg]

will follow this with a PR to make OpenMP optional (only required in the smallscaling benchmark) and probably make building the benchmarks optional as a whole (the gmake build does not include them by default either)