Align useflags of heavyweight packages in early image build with profile

[jonesmz]

Our genpi64 profile sets some useflags on gcc and other heavyweight packages that are different from the default from upstream gentoo.

Because our image build starts with a recent stage3, and then does

update portage
install ccache
update world
emerge --emptytree system
emerge --emptytree world # to ensure that transitive rebuilds from system, such as compiler updates, force non-system packages to be rebuilt with the right settings
set profile
emerge --update --newuse world

We can POTENTIALLY see gcc recompiled 3 times with different outcome.

If we set the limited list of use flags on the update world / emptytree system / emptytree world, like pgo, that result in gcc being different, then we can reduce the number of places where gcc gets recompiled, and see it instead just be installed from binpkg.

Really this just amounts to adding "USE=pgo" to the front of the emerge commands.