[new] lang/bootgcc

From GCC 11 onward, the codebase will require C++11. This poses a
problem for our gcc4 architectures. As discussed some years ago with
sthen@, here's a very minimal port of gcc8, containing only C, C++ and a
static libestdc++, installed under a different prefix, with the sole
purpose of bootstrapping gcc>=11. It will be used only in case we do
not have an ada bootstrap available.

Tested on hppa and sparc64. My alpha is too slow.