Do not supply `-mcpu` if `-optlc` provides `-mcpu` already.
Do not supply `-mcpu` if `-optlc` provides `-mcpu` already.

Reviewers: bgamari

Subscribers: thomie, carter

GHC Trac Issues: #14982

Differential Revision: https://phabricator.haskell.org/D4548

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc