In a later change, we will generate shlib-depends cross subpkgs during
pre-pkg stage. Thus we need shlib-provides information of all subpkgs
ready before pre-pkg is run.
Those information can only be read in post-install stage at the
eariliest.
Let's move the shlib-provides to post-install. This hook requires
prepare-32bit, so, let's move that hook, too.
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