Clang: Silence warnings (1/3)

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

Change-Id: I2b9970b932b51919d0382a2d4ed90d1ac793a7d2
Singed-off-by: Paul Beeler <sparksco@gmail.com>