Make tablesNextToCode a proper dynamic flag (#15548)

Summary:
There is no more use of the TABLES_NEXT_TO_CODE CPP macro in
`compiler/`. The default value of `tablesNextToCode` is calculated as
before, but now users of the GHCI API can modify this flag.

Reviewers:

Subscribers: TerrorJack, rwbarton, carter

GHC Trac Issues: #15548

Differential Revision: https://phabricator.haskell.org/D5082