[gentoo-amd64] eselect-compiler and multilib toolchain support

[Jeremy Huddleston]

So we're one step closer to getting multilib toolchain to the point
where we can drop ABI and base everything off of CHOST.  I just
committed eselect-compiler to portage (in package.mask) which will be
replacing gcc-config.  If you'd like to test it out, add the following
to /etc/portage/package.unmask:

app-admin/eselect-compiler
sys-devel/gcc-config

Most of the gcc-config commands should be the same, but you have a bit
more control using 'eselect compiler'.  If you've emerged gcc within the
last week, then you'll have the multilib profiles automatically.  If
not, the upgrade process tries to set it up for you automatically.  It
works fine for the normal user, but it's not smart enough to figure out
how to create the multilib profiles, so you'll either have to re-emerge
gcc or edit the configs in /etc/eselect/compiler yourself (see attached
examples).

If you edit the configs manually, do a 'eselect compiler update' to make
sure the wrappers are up to date.  This will result in you having an
i686-pc-linux-gnu- prefixed compiler set rather than using 'gcc32' or
remembering the '-m32' in CFLAGS.  You can also use a separate version
and specs for the x86 and amd64 toolchains this way.

Please test this out, break it, and send me feedback and patches.  Once
this gets cleaned up and hammered out, we can do the same for
binutils-config as well...

Thanks,
Jeremy