Summary: We should escape path to ghc.On wondows usually ghc comes from HP, which is installed somewhere in "...\Haskell Platform\..." Note space in the middle.

Test Plan: not necessary

Reviewers: rwbarton, hvr, austin

Reviewed By: rwbarton, hvr, austin

Subscribers: rwbarton, simonmar, ezyang, carter, thomie

Projects: #ghc

Differential Revision: https://phabricator.haskell.org/D304