Hadrian: support stage1 profiling
This adds the profiled_stage1_ghc flavour transformer, which allows developers to compile GHC with profiling enabled for the stage 1 compiler, e.g. with the Hadrian invocation:
hadrian/build --flavour=devel1+profiled_stage1_ghc
This is useful when one is debugging the stage 1 compiler, e.g. because the compiler can't build base or the stage 2 compiler.