ARMv4 (not v4t) marked obsolete in gcc-6

[Arnd Bergmann]

I've found out that ARMv3 and ARMv4 is now on track to get removed from gcc in the future,
so I'm trying to alert everyone that I have knowledge of using ARMv4 based platforms that
we currenly support in the Linux kernel.

The architecture has been declared obsolete here: https://gcc.gnu.org/gcc-6/changes.html
and it will be removed in the following release (gcc-7) one year later, unless someone
raises concerns over it.

We will of course be able to compile kernels for a long time using older compilers, but
this tends to get harder over the years as people upgrade to newer distros.

Here is an overview of which ARMv4 platforms we still have as of Linux-4.6:

* Moxart: this is the only one that was recently (2013) added, and is apparently
  hardware that remains commercially available.

* Gemini: officially supported in OpenWRT today, with the latest compiler. This one
  will likely cause the most issues for actual users. It would be helpful to get
  some numbers about users or downloads here, to see whether it can be dropped
  in a future OpenWRT release or if it might be possible to leave this on
  gcc-6.x when the other platforms move on to gcc-7+

* sa1100: A lot of people have these, but I'm guessing this is mostly interesting
  for hobbyists that are able to keep using older gcc versions.

* RiscPC, Footbridge, EBSA110: Classic systems that used to be popular in
  the past but only remain in small quantities as far as I know. Russell still
  uses them. He also uses older compilers, so probably isn't affected
  immediately.

	Arnd