[LLD] [COFF] Add a private option for setting the os version separately from subsystem version

The MinGW driver has separate options for OS and subsystem version.
Having this available in lld-link allows the MinGW driver to both match
GNU ld better and simplifies the code for merging two (potentially
mismatching) arguments into one.

Differential Revision: https://reviews.llvm.org/D88802