Add -Wno-unused-command-line-argument to clang cflags to enable ccache.

Previously ccache is disabled when it fails calling clang's preprocessor with
unused arguments (such as '-Wa,--noexecstack') in the command line.

See http://petereisentraut.blogspot.com/2011/05/ccache-and-clang.html.
(-Qunused-arguments suppresses more than
-Wno-unused-command-line-argument does.)

Change-Id: I6cde307632c8395c053eb28063d7844d93070562