pgo: profile guided optimization with gcc

pgo.sh

# Instrument binaries, pgo data to /data/pgo, serial make is important to not confuse the pgo generator
env CXXFLAGS='-march=native -fprofile-dir=/data/pgo -fprofile-generate=/data/pgo' cmake .. -DCMAKE_BUILD_TYPE=Release                   
make -j 1

# Run instrumented program, generate and write pgo data
./runIt

# Use profile data and feed into gcc, correct for threading counter noise, serial make is important to not confuse the pgo generator
env CXXFLAGS='-march=native -fprofile-dir=/data/pgo -fprofile-use=/data/pgo -fprofile-correction' cmake .. -DCMAKE_BUILD_TYPE=Release                   
make -j 1