libstdc++-v3 won't build on IRIX 5.2

[Benjamin Kosnik]

> Should I leave config/irix/bits as-is, or move it to
> config/irix/irix6.5/bits?

make 

config/irix/irix6.5
config/irix/pre-irix6.5

or just do versions for each release, as appropriate.

-benjamin