This patch provides user with a better hint when most RTS options
are not available (not compiled with `-rtsopts`).

A new field "rtsOptsEnabled" is added into RtsFlags.MiscFlags to
tell the availablity of RTS options.

Some concerns:
* Unlike other flag fields in "libraries/base/GHC/RTS/Flags.hsc",
  "RtsOptsEnabled" is defined in "includes/RtsAPI.h" and lacks
  constant macros. Therefore In "GHC.RTS", "RtsOptsEnabled" simply
  derives Enum instance and reads as of type "CInt".

* There are other ways to change RTS options (e.g. `-with-rtsopts`),
  but it might be too verbose to mention.

Test Plan: validate

Reviewers: austin, hvr, thomie, simonmar

Reviewed By: thomie

Subscribers: thomie, rwbarton

Differential Revision: https://phabricator.haskell.org/D767

GHC Trac Issues: #9579