[gentoo-dev] Building hardened gcc specs always, just not enabling them by default

Looks like the thread I started about moving more hardened features to
default
<http://archives.gentoo.org/gentoo-dev/msg_ef3dbd4ba400a5936cd5b7546b86d875.xml>
got a lot of positive feedback. Kernel hardening features are more
problematic, but hardening the toolchain seems to be within reach.

I'd like to produce some implementation plan for that, and my suggestion
is to change the meaning of the "hardened" USE flag for GCC. I'd like to
build all 4 or so specs for gcc always, and the "hardened" USE flag
would just control which one is the default: the vanilla one or
full-hardening one.

This would allow people to manually start using hardened toolchain
without even switching profile, and should be a no-op for everyone else.
From there we can later proceed to apply more features.

Thoughts?