[PECOFF] Set default subsystem to the DLL header.

Subsystem field in the PE/COFF file header has no meanining for the DLL.
It looks like MSVC link.exe sets the default subsystem (Windows GUI) to
the field if no /subsystem option is specified.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@198015 91177308-0d34-0410-b5e6-96231b3b80d8