Utility: ignore OS version on non-Darwin targets in `ArchSpec`

Utility: ignore OS version on non-Darwin targets in `ArchSpec`

The OS version field is generally not very helpful for non-Darwin
targets.  On Linux, it identifies the kernel version which moves
out-of-sync with the userspace.  On Windows, this field actually ends up
corresponding to the Visual Studio toolset version instead of the OS
version.  Consider non-Darwin targets without an OS version to be fully
specified.

Differential Revision: https://reviews.llvm.org/D88181
Reviewed By: Jonas Devlieghere, Dave Lee