This completes a5227080, the
ghci-usage.txt file are also used by the
library so need to make sure they are present in the libdir even if we
are not going to build
This also fixes things for cross compilers because the stage2 cross-compiler requires the ghc-usage.txt file, but we are using the stage2 lib folder but not building stage3:exe:ghc-bin so ghc-usage.txt was not being generated.