BaseTools GnuMakefile: Update GCC Flags to the specific one with BUILD_ prefix

BaseTools GnuMakefile: Update GCC Flags to the specific one with BUILD_ prefix

To avoid the conflict with the default GCC flag name, BUILD_ prefix is added.

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: Yonghong Zhu <yonghong.zhu@intel.com>
Reviewed-by: Giri P Mudusuru <giri.p.mudusuru@intel.com>