distrib/configure: Fail if we can't detect machine's word size

This is a sure sign that something is terribly wrong.

We also now verify that the word size that the binary distribution
expects matches the word size produced by the local target toolchain.

Finally we rename WordSize to TargetWordSize, since non-host/target
qualified quantities are terribly confusing.

Reviewers: austin, hvr, Phyx

Reviewed By: Phyx

Subscribers: Phyx, rwbarton, thomie, erikd

Differential Revision: https://phabricator.haskell.org/D3711