PS: I'm not asking for someone to post patches to configure.in or what ever,
(hinting maybe), but I am asking for some pointers on where to look.
This would be very tricky to do. GDB links to libraries built in the
same tree, so nothing in gdb/configure.in would work. It would have to
be in the top level, which is shared with binutils and gcc and so on.
You don't want them to default to being 64-bit apps, because they
should obey the system policy of defaulting to 32-bit.
I really think this is your responsibility when building GDB if you
want it to support 64-bit binaries.