rts: Don't build StgCRunAsm.S if unregisterised
rts: Don't build StgCRunAsm.S if unregisterised

StgCRunAsm.S provides StgCRun on powerpc64le platforms when
registerised.  However, in the unregisterised setting we use the
mini-interpreter and consequently shouldn't build StgCRunAsm.S lest we
get duplicate symbols.

Test Plan: Build unregisterised compiler on AIX.

Reviewers: hvr, trommler, austin, simonmar

Reviewed By: trommler, simonmar

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3560

ezyang/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc