Linux sigtramp detection code moved to its proper place

[Eli Zaretskii]

>   What would you suggest for a non-buggy Tar program to do, exactly?
>
> Simply overwrite the files, possibly asking confirmation from the
> user.  This shouldn't be a problem, since the DJGPP native port
> doesn't need i386-linux-nat.c nor i386-linux-tdep.c.

The users don't know whether these files are or aren't needed.  So
they might overwrite the wrong files, and mess up the build beyond
any recognition.