[PECOFF] Set DLL bit in PE header if DLL.

Windows loader can load a DLL without this bit but it wouldn't
call the initializer function in the DLL if the bit is absent.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@213216 91177308-0d34-0410-b5e6-96231b3b80d8