add a 'threaded' flag for hadrian

Authored by alpmestan on Oct 1 2018, 8:14 AM.

Description add a 'threaded' flag for hadrian

hadrian will explicitly enable this flag, but more importantly needs it
as otherwise we just never end up passing -threaded to GHC when building
a new GHC binary. We could quite likely unconditionally pass -threaded,
as the corresponding logic for GhcThreaded in mk/ seems to
always lead to it being set to True, but we instead leave a way out for
anyone in need of a GHC linked against a non-threaded runtime system in
the future.

Test Plan: T8242 (with a GHC built by hadrian)

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, carter

Differential Revision: