PGO should be the default for release build

[k-sareen]

We should take the most important plans and profile them for the "release" build. Users of MMTk should get the best performance for "release" mode as opposed to having to do extra work with the PGO build themselves. Important plans are ones such as Immix, Compressor, GenImmix, etc.

[qinsoon]

I imagine that it may not be easy to achieve this with the build system.

To do a PGO build, we will need to finish the entire build first. Then there needs to be a step to run the build with certain parameters, and certain paths to the workload. Then redo part or all of the build process to do another build for a different MMTk library with the profiles.

This would involve a lot of changes to the current build script, while it is much easier to use an external script like pgo-build.sh to drive a PGO build.

One alternative could be that we produce a 'standard' profile (which could be generated by CI) and include the standard profile with each commit. In this case, it would be much easier and natural to integrate a PGO build with the build script.