lib/libc/Versions.def: new symbol version for 12.x

[Ed Maste]

On 29 July 2016 at 14:53, Daniel Eischen <deischen at freebsd.org> wrote:
>
> New symbol versions can be rolled back to -stable or other prior
> baselines.

If that happens I think having an FBSD_12.0 symbol in stable/11
actually makes it more clear what's happening, than if a FBSD_1.5
symbol appears in stable/11.

> Also, if there are no changes in a major release branch,
> there's no need to bump the symbol version.

That's fine too, the version would just jump from FBSD_10.0 to
FBSD_12.0 for example.