Clang: Silence warnings (3/3)

With GCC >=4.7 Clang generates a lot of noise with various compiler flags,
and can even break the builds when using Flags like Graphite.  This can be
reproduced by doing the following:

Setting TARGET_EXTRA_CFLAGS with Graphite flags such as these
TARGET_EXTRA_CFLAGS +=  -fgraphite-identity \
                        -floop-block \
                        -floop-strip-mine \
                        -ftree-loop-distribution \
                        -ftree-loop-linear

Causes the following errors to occure and the builds to fail:

clang: error: argument unused during compilation: '-fgraphite-identity'
                                                  '-floop-block'
                                                  '-floop-strip-mine'
                                                  '-ftree-loop-distribution'
                                                  '-ftree-loop-linear'

Silence the errors by passing the -Qunused-arguments for CFLAGS

Singed-off-by: Paul Beeler <sparksco@gmail.com>