Fix build on amd64/solaris.

Summary:
Commit 71fcc4c0 breaks the 64bit build on Solaris 11. Solaris
is a multi-lib OS so both 32bit and 64bit binaries may  be run, but
by default it compiles to 32bit so that -m64 needs to be added in
the appropriate place when compiling for 64 bits.

Patch-from: Karel Gardas <karel.gardas@centrum.cz>

Reviewers: kgardas, austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D733

(cherry picked from commit 83afcd17)