ABI defaults

[Torbjorn Granlund]

We need to establish a compiler before we can make any sort of detection
of the "default ABI", since the default ABI is per compiler.  Trying to
determine any "system default ABI" will *not* work robustly.

This means that we need to put this new functionality after the big evil
cpu-os switch in configure.in, after we've determined CC but before
we've determined ABI.

We currently try compilers until we find one that supports the best ABI.
We need to disable that logic for Niels' proposed feature.

-- 
Torbjörn