Fix gas/22304 by forcing a 64-bit bfd. Suggest the same for other targets.

[Hans-Peter Nilsson]

> Date: Sun, 22 Oct 2017 13:23:07 +0200
> From: Hans-Peter Nilsson <hp@axis.com>

> Maybe it's a good idea to force bfd64 for all targets: remove the
> bfd64 machinery equivalent to setting it to true, as you can get bad
> code from the buggy expression handling.  Though, I guess you can
> construct a just-as-failing 64-bit test for bfd64...
> 
> Here's a generic reproducer test-case taken from one of the failing
> tests, that will fail for all 32-bit-bfd builds.

I opened PR gas/22335 for this.

brgds, H-P