[PECOFF] Minimum size of DOS stub is 64 byte.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@194791 91177308-0d34-0410-b5e6-96231b3b80d8
Fix Windows buildbot on which size_t is not unsinged long.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@194793 91177308-0d34-0410-b5e6-96231b3b80d8