[arch-dev-public] C++ ABI bump rebuild

[Allan McRae]

Hi everyone,

GCC-5.x release libstdc++ with a dual ABI [1].  The time has come to
rebuild everything C++ related to the new one!

This means:
 - do not use [staging] for anything else
 - do not update anything C++ until it is done
 - help fixing anything that does not build...

We will use foutrelis rebuild tool to do the building.  This means TUs
can not help building (the tool does not separate out [community] from
[core] and [extra] yet), but you can fix build failures. Keep an eye on
[2] for the build status.

The build will start as soon as I get gcc ready.

[1] https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_dual_abi.html
[2] https://rebuilds.foutrelis.com/

Allan