Emitter: guard _RAND_* declarations with ifdef (bp #1548) (#1553)

* Emitter: guard _RAND_* declarations with ifdef (#1548)

* Emitter: add declare functions ifdef guard

* Emitter: add ifdef initials

* Emitter: add comments, cleanup

* Emitter: changes from code review

- make new methods private
- use .withDefault
- remove empty initial block

Co-authored-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>
(cherry picked from commit 3b4e691b

)

* Emitter: revert Unit result type to ArrayBuffer (#1557)

Co-authored-by: Albert Chen <40366337+albertchen-sifive@users.noreply.github.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>