Renaming mingw64 to mingw* for x86_64 architecure

[DJ Delorie]

>   Am I correct in assuming that your synchronisation scripts will
>   bring this change into the sourceware repository, or should I apply
>   the patch there as well ?

The toplevel scripts only advise the build maintainers that they're
out of sync.  We have to manually sync those because they're
bidirectional and sometimes messy, and I haven't had the time or
desire to write the scripts needed to automate it.

The libiberty/ and include/ scripts do everything but the final commit
automatically (I have to push the "yes, please" button manually), and
always from gcc->src.