Allow CmmLabelDiffOff with different widths

Summary:
This change makes it possible to generate a static 32-bit relative label
offset on x86_64. Currently we can only generate word-sized label
offsets.

This will be used in D4634 to shrink info tables.  See D4632 for more
details.

Test Plan: See D4632

Reviewers: bgamari, niteria, michalt, erikd, jrtc27, osa1

Subscribers: thomie, carter

Differential Revision: https://phabricator.haskell.org/D4633