libstdcxx_so_7 branch proposal

[Benjamin Kosnik]

Hey. I'd like to tag and branch libstdc++-v3 on mainline, so that the
libstdc++ hackers have a place to stash ABI-incompatible changes.

Note, there are no such changes now, and there remains a plan to keep
libstdc++ ABI's compatible between 3.4/3.5, but I think this branch will
help with logistics, and will give us more options for managing change
in the future.

I'll wait a day for comments.

-benjamin