Add -falignment-sanitization flag

Here we add a flag to instruct the native code generator to add
alignment checks in all info table dereferences. This is helpful in
catching pointer tagging issues.

Thanks to @jrtc27 for uncovering the tagging issues on Sparc which
inspired this flag.

Test Plan: Validate

Reviewers: simonmar, austin, erikd

Reviewed By: simonmar

Subscribers: rwbarton, trofi, thomie, jrtc27

Differential Revision: https://phabricator.haskell.org/D4101