[Dlang-internal] (De)Duplication of information in DRuntime

[Walter Bright]

The general idea for OS-specific files is to *not* attempt to merge them.

The problem with merging them, is when someone doing a PR for, say, Linux, 
changes something in the common part, they can (and inevitably will) silently 
and disastrously break some other OS.

Hence, PRs for Linux headers should *only* affect Linux systems.

This works because Linux headers are never used by FreeBSD systems, even if they 
have identical sections.