Hadrian: support in-tree GMP

Summary:

This adds top-level configure flags --with-intree-gmp and --with-framework-preferred, both of which are especially relevant on MacOS.

Besides gaining two new flags, Hadrian also had to be taught what to do with the framework in .cabal files.

Test Plan:

./boot && ./configure --with-intree-gmp && ./hadrian/build.sh
./boot && ./configure --with-gmp-framework-preferred && ./hadrian/build.sh # on macos

Reviewers: carter, snowleopard, alpmestan, hvr, goldfire, bgamari

Subscribers: rwbarton, erikd

GHC Trac Issues: #16001 (closed)

Differential Revision: https://phabricator.haskell.org/D5417