BaseTools GNU makefile: Add BUILD_CXXFLAGS to align make built-in rule

BaseTools GNU makefile: Add BUILD_CXXFLAGS to align make built-in rule

GNU make built-in rule to Compiling C++ programs with
‘$(CXX) $(CPPFLAGS) $(CXXFLAGS) -c’.
To align to it, add empty BUILD_CXXFLAGS in cpp rule.

Cc: Giri P Mudusuru <giri.p.mudusuru@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Giri P Mudusuru <giri.p.mudusuru@intel.com>
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>