Only use -fasm on platforms with an NCG (Closes: #9884).

Summary: Signed-off-by: Erik de Castro Lopo <erikd@mega-nerd.com>

Reviewers: austin, carter

Reviewed By: carter

Subscribers: carter, thomie

Differential Revision: https://phabricator.haskell.org/D570

GHC Trac Issues: #9884