A Fresh Look At The PGO Performance With GCC 8

The PGO performance benefit was of small benefit in the three matrix multiplication to not much of a difference in the two other sub-tests.

With the AOBench ambient occlusion renderer, the PGO-optimized build was of a small but noticeable advantage.

Similarly, with the M-Queens OpenMP benchmark was a small performance boost on top of what's squeezed out by already using "-O3 -march=native" for compiler optimizations.

The C-Ray multi-threaded ray-tracer also benefited from the Profile Guided Optimizations on GCC 8.