Export performBlockingMajorGC from System.Mem

There was originally an MR to introduce performBlockingMajorGC !3389. This both implemented a C function with this name and exported a Haskell wrapper from System.Mem.

Some of this was merged in the form of ba73a807. Yet, the export from System.Mem was dropped.

I think nowadays this would require a CLC proposal. Perhaps this is the reason why it was dropped? Or perhaps this was just an accident, or perhaps there's a different reason. @bgamari might know?

I'd be happy to open a CLC proposal and MR to export this from System.Mem.