Update link job for flang on windows

When linking a Fortran program, we need to add the runtime libraries to
the command line. This is exactly what we do for Linux/Darwin, but the
MSVC interface is slightly different (e.g. -libpath instead of -L).

We also remove oldnames and libcmt, since they're not needed at the
moment and they bring in more dependencies.

We also pass `/subsystem:console` to the linker so it can figure out the
right entry point. This is only needed for MSVC's `link.exe`. For LLD it
is redundant but doesn't hurt.

Differential Revision: https://reviews.llvm.org/D126291



Co-authored-by: Markus Mützel <markus.muetzel@gmx.de>