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

[Roland McGrath]

> Unfortunately, this can't be done.  We must really set CC/CFLAGS to
> the proper value before we run any configure tests whatsoever,
> otherwise results may end up incorrect.

The builder is always expected to provide the baseline CC and CFLAGS
necessary to produce code for the platform.  I don't think you should have
platform configure magic for this.  That is independent of how libc's
sysdeps selections work.

The sysdeps selections are based on the tuple out of config.sub, and that's
as it should be.  If you have a different ABI, that is a different
configuration tuple for the purposes of GNU configure scripts.