Fix file-check-cxx to handle quoted arguments

If the OVERRIDE_C_DATE_TIME environment variable is set, the build
system will add a macro to the global cflags that contains a static
quoted date, like:

  -D__DATE__="\"Sep 28 2015\""

file-check-cxx needs to properly pass that through as a single, still
quoted argument. So use the quoted "$@" instead of turning each word
into an argument.

Change-Id: I6929d712c280d0452062640e0cba8a176d045c97