enable n32 and n64, and move o32 into mips/mips32

[Roland McGrath]

> The unfortunate thing about it is that, when you have a
> mips64-linux-gnu cross toolchain, if you specify say
> --host=mips64-linux-gnu-abin64, configure won't automatically find
> mips64-linux-gnu-gcc for you, like it does for
> --host=mips64-linux-gnu.  That's the reason I wanted to be able to
> detect the ABI from CFLAGS.  

Tough beans.  The same is true of sparc or x86 with -m32 and -m64, etc.
The config.sub canonical tuple is used as a key to many things where it
must correspond exactly to an ABI--that's what it's for.  It is by far more
reasonable to add compiler flags based on the tuple than to use compiler
settings instead of just the tuple to choose the configuration.  The latter
is just impractical.  The former I object to on the grounds of consistent
GNU configure conventions.  Those require that $CC as given/defaulted be a
compiler that produces correct code for the host tuple given/defaulted.  If
you want to get AC_PROG_CC changed in Autoconf so that it does some mips
hooey universally, I would not object to that.