When cross-built GHC is being installed one of
latest steps is to register installed libraries
with 'ghc-pkg'.

GHC uses freshly installed 'ghc-pkg' and 'ghc-stage2'
for that.

Tested as:
    ./configure --target=aarch64-unknown-linux-gnu
    make install DESTDIR=$(pwd)/__s2 STRIP_CMD=:

Before the change install failed on ghc-pkg execution phase:

    ".../ghc-cross/__s2/usr/local/lib/ghc-8.3.20170406/bin/ghc-pkg" \
        --force \
        --global-package-db \
        ".../ghc-cross/__s2/usr/local/lib/ghc-8.3.20170406/package.conf.d" \
        update rts/dist/package.conf.install
    /bin/sh: .../ghc-cross/__s2/usr/local/lib/ghc-8.3.20170406/bin/ghc-pkg: \
        No such file or directory

To avoid breakage we use 'ghc' and 'ghc-pkg' built by stage0.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

Test Plan: run 'make install' on stage2 crosscompiler

Reviewers: rwbarton, austin, bgamari

Subscribers: thomie, snowleopard

Differential Revision: https://phabricator.haskell.org/D3432