Go C++ only

Hi all,

As last discussed, the general consensus was to make 7.12
the last gdb version that supports building with a C compiler.

I've been holding proposing to drop C-mode support in master
with the idea that that would help with all the bugfix
backporting going on.  I've had to explain this several times
recently, as people are eager to use C++ features.  Multiple
times this week, even.

At this point the backporting rate is quite low though, so
maybe it no longer makes that much sense to hold C++ back,
at least in new code.

What do people think?  Time to march ahead?

Thanks,
Pedro Alves