Reorder archname-related questions in configure.com.

Reorder archname-related questions in configure.com.

By moving these to after other questions have been asked, we can
construct a proper $Config{archname} that indicates threads, long
doubles, multiplicity, or anything else that needs to be appended
to the architecture name.

This means letting go of the ability to configure for a different
architecture than the one you're running on, but that feature is
unlikely to have worked very well in recent years anyway as there
have been an increasing number of features that are not available
on all architectures.