Add a module allowing easy use of Profile Guided Optimization (PGO)

It would be useful to have a standard PGO workflow in cmake for GCC/Clang/Visual Studio.

It has been requested on the mailing list a couple of times

• https://cmake.org/pipermail/cmake/2018-October/068518.html

There appear to be a few bits on GitHub that might be suitable for a basis

• https://github.com/chrisjwilliams/lofarburst/blob/d516e8c85dfa35c99e0f2a6640d3e9f632683aa0/cmake/profile_guided_optimization.cmake
• https://github.com/jdm7dv/Microsoft.NET/blob/f46cf8a239e94dbf298e04c1c164f9f13c05753c/coreclr/pgosupport.cmake