Make tablesNextToCode a proper dynamic flag (#15548)

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

That said, GHC still is hardcoded to define TABLES_NEXT_TO_CODE based on
that default value. This is bad, but neccessary until the remaining uses
of TABLES_NEXT_TO_CODE get it from make/Hadrian.

Reviewers:

Subscribers: TerrorJack, rwbarton, carter

GHC Trac Issues: #15548

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