Interpreter.c: use macros to access/modify Sp

This is another step in fixing #13825 (based on D38 by Simon Marlow).

This commit adds a few macros for accessing and modifying `Sp`
(interpreter stack) and will be useful to allow sub-word
indexing/pushing. (but that will be a separate change, this commit
should introduce no changes in behavior)

Signed-off-by: Michal Terepeta <michal.terepeta@gmail.com>

Test Plan: ./validate

Reviewers: bgamari, simonmar, austin, erikd

Reviewed By: bgamari, erikd

Subscribers: rwbarton, thomie

GHC Trac Issues: #13825

Differential Revision: https://phabricator.haskell.org/D3744