FL-1832: initial abi_* flag support for multilib gcc

Replace semi-opaque ABI control via USE=multilib and profile variables
with fine-grained control via abi_* flags. Currently this is pretty much
a visual difference but we can add proper bootstrapping support in the
future and allow users to dynamically switch multilib ABIs in gcc.
gcc-4.9.2-r1: use autodetecting compiler symlink creation from ::mgorny

Replace the compiler symlinking code based on toolchain.eclass with the
one used in ::mgorny overlay. Instead of using a hardcoded list of
tools, the new code simply globs for all tools installed. It also
replaces a few hardlinks installed by gcc with symlinks.
FL-1832: install gcc wrappers for other multilib ABIs

Install gcc wrappers and gcc-config files to support using host gcc as
an alternate ABI compiler. For example, gcc on multilib amd64 can be
used as i686-pc-linux-gnu compiler. This makes it possible to use
distcc on an amd64 host for x86 clients. It will also make multilib
cleaner in the future.
gcc-4.9.2-r1: stub out fixing includes

Stub out the include fixing script instead of removing all the files
created by it.
Merge remote-tracking branch 'mgorny/mgorny-improvements'

This adds various good general ebuild cleanups as well as more fine-grained
ABI support from mgorny.