BaseTools: Fix the Windows GCC Build Failure with too long path

BaseTools: Fix the Windows GCC Build Failure with too long path

When path is too long, build tool will wrap them into resp.txt, then call
gcc @resp.txt. It will cause windows GCC build failure, because resp.txt
still uses windows directory separator \. This patch change the \ to /.

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