Add the ability to specify subsystem version when linking PE executables

[Aransentin]

LLD specifies version 6 by default, i.e. Vista: https://github.com/llvm-mirror/lld/blob/8c742da1fd4f4acc78032a34021de228e3621ea5/COFF/Config.h#L205

This can however be overwritten by specifying the minor/major version after the subsystem name, like so: https://github.com/llvm-mirror/lld/blob/4fb31540fb812c46e3f8d2b6b7e4115ed0930693/COFF/DriverUtils.cpp#L112

Specifying the version yourself might be useful if you're using OS features available in later versions than Vista, or earlier versions if you are making demos for older systems.