accessors to RTS flag values -- #5364

Summary: Implementation of #5364. Mostly boilerplate, reading FILE fields is missing.

Test Plan:
- Get some feedback on missing parts. (FILE fields)
- Get some feedback on module name.
- Get some feedback on other things.
- Get code reviewed.
- Make sure test suite is passing. (I haven't run it myself)

Reviewers: hvr, austin, ezyang

Reviewed By: ezyang

Subscribers: ekmett, simonmar, ezyang, carter, thomie

Differential Revision: https://phabricator.haskell.org/D306

GHC Trac Issues: #5364

Conflicts:
	includes/rts/Flags.h
Minor tweaks to API Annotation

Summary:
Add missing Outputable instance for AnnotationComment

Update documentation

Adjust parser to capture annotations correctly

Test Plan: ./validate

Reviewers: austin

Reviewed By: austin

Subscribers: thomie, carter

Differential Revision: https://phabricator.haskell.org/D520