build_image_util: do not symlink etc/portage/profile

Create profile as a real directory instead of a symlink to the board
root's configuration. Normally the board root does not modify this but
it is useful for build_image to use it to modify package.provided.
prod_image_util: install GCC libraries to /usr/lib

Normally GCC is installed in a way that allows installing multiple
versions and switching between them. Our production images do not need
this and additionally the only things from the GCC package that are
needed are the shared libraries. To ensure these libraries are *always*
locatable regardless of the presence of /etc/ld.so.conf and
/etc/ld.so.cache we can install those libraries to plain old /usr/lib.
The GCC packages don't have a built in way to do this but we can get
away with extracting the libraries directly from the binary package.

This is actually similar to what ChromeOS did with a few exceptions:
 - We use a native GCC build instead of the cross toolchain
 - The archive is properly extracted from the package instead of feeding
   the package directly to tar and ignoring the resulting warnings.

As an added benefit switching from a blacklist to a whitelist ensures
that extra cruft does not slip through the cracks, saving 5-10MB.
Merge pull request #323 from marineam/gcc

Make ldconfig not matter so much