UNREG: ignore -fllvm (Trac #13495)

Unregisterised GHC can only use C as a target backend
(option used to be called -fvia-C). -fasm option was
ignored with a warhing, but not -fllvm.

jms noticed the failure when tried to use quick-cross
build flavour. quick-cross enables -fllvm in makefile.

  "inplace/bin/ghc-stage1" ... -fllvm
  ghc-stage1: panic! (the 'impossible' happened)
    (GHC version 8.0.2 for powerpc-unknown-linux):
          LlvmCodeGen.Ppr: Cross compiling without valid target info.

This change ignores -fllvm as well.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
(cherry picked from commit 74615f41)