Fix building issue for VS2003 and VS2005 tool chain. The "/GS-" is not recognized by VS2003 but need by VS2005, so the CFLAGS should override from tools_def.txt but not replace CFLAGS defined in tools_def.txt.
Fix building issue for VS2003 and VS2005 tool chain. The "/GS-" is not recognized by VS2003 but need by VS2005, so the CFLAGS should override from tools_def.txt but not replace CFLAGS defined in tools_def.txt.

git-svn-id: 5648d1bec6962b0a6d1d1b40eba8cf5cdb62da3d@7586 6f19259b-4bc3-4df7-8a09-765794883524

MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2