ghc-deps.sh: also support ghc-pkg for ghc build before 7.6
    
    Now test for "%buildroot%_bindir/ghc-pkg-$GHC_VER" and if it exists it means
    we are doing a ghc build and then condition on GHC_VER to set GHC_PKG.
    Otherwise just use %_bindir/ghc-pkg-$GHC_VER as usual.
    
    There are 3 cases: 7.8, 7.6, and earlier.