[PECOFF] Make /disallowlib an alias for /nodefaultlib.

I'm not sure if it is really an alias for /nodefaultlib, but I can say that
they are at least similar. Making it an alias would be better than ignoring it.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@194131 91177308-0d34-0410-b5e6-96231b3b80d8