BaseTools/tools_def: suppress GCC predefined macros in DTB compilation

BaseTools/tools_def: suppress GCC predefined macros in DTB compilation

The standard GCC preprocessor we use to preprocess device tree source
files has a whole bunch of macros predefined, among which

  #define __linux 1
  #define __linux__ 1
  #define __gnu_linux__ 1
  #define linux 1

This causes a property like 'linux,code' to be converted into '1,code'
which is obviously wrong. So let's get rid of all the predefined macros
by passing -undef to the preprocessor command line.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>