For now, for single-threaded GCN, nvptx target use only; extension for
multi-threaded offloading use is to follow later. Eventually switch to
libstdc++-v3/libsupc++ proper.
libgcc/
* c++-minimal/README: New.
* c++-minimal/guard.c: New.
* config/gcn/t-amdgcn (LIB2ADD): Add it.
* config/nvptx/t-nvptx (LIB2ADD): Likewise.