catalyst: update GCC dependencies before building stage1

When bootstrapping a SDK we need to update GCC dependencies to ensure
the GCC built for stage1 is linked against the same library versions as
those that are included in the stage1. Without this updating the mpc
library just results in a broken stage1.
catalyst: make sure the 'portage' user can write to distfiles and ccache

Adding the update step appears to break permissions on the distfiles
directory. Ensure the portage user is correct and set the permissions on
directories it needs to write to in advance.