[llvm-objcopy] Implement the PE-COFF specific --subsystem option

This implements the parsing of the highly PE-COFF specific option
in ConfigManager.cpp, setting Optional<> values in COFFConfig, which
then are used in COFFObjcopy.

This should fix https://github.com/mstorsjo/llvm-mingw/issues/239.

Differential Revision: https://reviews.llvm.org/D116556