Recent changes (commit 0d1a8d09) for 64 bit platforms allowed GHC
to mmap one huge (currently 1 terrabyte) memory region from which to
do its own allocations. This is enabled by default, but it would be
nice (even just from the point of view of testing) to be able to
disable this at configure time.

Test Plan: configure and grep mk/config.h for USE_LARGE_ADDRESS_SPACE

Reviewers: austin, ezyang, bgamari, rwbarton

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1170

GHC Trac Issues: #10791