Use common_clang_cflags

Use common_clang_cflags instead of adding to common_cflags if
LOCAL_CLANG==true

This makes the construct work with USE_CLANG_PLATFORM_BUILD=true
(and is simply nicer)

Change-Id: I4797ac737c8e57e8cefb8558ee5f5387ef035931
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>
Merge "Use common_clang_cflags"