BaseTools: fix the bug for [BuildOptions] of multiple workspace support

BaseTools: fix the bug for [BuildOptions] of multiple workspace support

when enable Multiple workspace and there have other option(eg: -I) before
$(WORKSPACE), handleWsMacro cannot return correct which cause the
ArmVirtPkg build failure.
example:
[BuildOptions]
  *_*_AARCH64_PLATFORM_FLAGS == -I$(WORKSPACE)/ArmVirtPkg/Include

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>