BaseTools/tools_def: use separate PP definition for DTC

BaseTools/tools_def: use separate PP definition for DTC

Clang's preprocessor behaves differently from GCC's, and produces
intermediate device tree source that still contains #pragma pack()
and other directives that the device tree compiler chokes on.

For assembling device tree sources, it matters very little which
preprocessor is being used, so let's just use GNU CPP explicitly.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>