linux(4): Retire linux_kplatform.

[dchagin]

Summary

Assuming we can't run on i486, i586 class cpu, retire linux_kplatform var
and use hardcoded 'machine' value in linux_newuname().

I have added linux_kplatform for consistency with linux_platform which is
placed in to vdso to avoid excess copyout it on stack for AT_PLATFORM at
exec time.

This is the first stage of Linuxulator's vdso revision.

[imp]

While FreeBSD can run on 486 and 586 class cores, Linux has long ago made the choice to compile for 686 and newer cores only in its popular distributions. FreeBSD changed its default to i686 for 13 as well.
One in theory could run a 486 or 586 binary on a machine of that class. However, that's such a odd-ball combination I'm virtually certain has seen no testing that this is fine.