configure + Irix64 problem

[Quinn Dunkan]

When compiling python on an indigo2, I ran up against a little problem:
`uname -s` returns 'IRIX64', which means python looks for platform-specific
modules in plat-irix646 and doesn't find any, so imports of FCNTL etc. fail,
and you have to manually hack the config.status (or whatever) file to change
MACHDEP to irix6.

I'm not sure if the python library should acquire plat-irix646, or if
configure should be modified to read IRIX64 as IRIX, but the current
behaviour is probably a bug...