GDBserver ports cleanup

[Simon Marchi <simon.marchi@polymtl.ca>]

Hi,

I would like to propose a cleanup in the stale / unused / outdated GDBserver
ports (the same could be done with GDB, but I'm tackling GDBserver for now).

It is a recurring theme that when doing changes in common functions, we need to
change files that we can't build.  We sometimes find blatant mistakes that wouldn't
even compile in these files, which shows that nobody is building them.  If nobody
is using them, I'd like to remove them, as it takes up some precious developer time
to consider them in our changes.  It also confuses people as to why we keep code
that doesn't build in our repo...

Looking at the *-low.cc files, here are the platforms GDBserver supports today:

- linux-aarch32
- linux-aarch64
- linux-arm
- linux-bfin
- linux-cris
- linux-crisv32
- linux-ia64
- linux-m32r
- linux-m68k
- linux-mips
- linux-nios2
- linux-ppc
- linux-riscv
- linux-s390
- linux-sh
- linux-sparc
- linux-tic6x
- linux-tile
- linux-x86
- linux-xtensa
- lynx-i386
- lynx-ppc
- nto-x86
- win32-arm
- win32-i386

The ones I'm thinking should go for sure are lynx (LynxOS) and nto (Neutrino).  As
far as I know, it's not possible to build GDBserver for these without having access
to non-publicly available toolchains/sysroots from the vendors, so it's not
reasonable to expect the community to maintain it.  And seeing that nobody made changes
specific to these ports in many years, I conclude that nobody is really using that code.
Of course, if somebody has access to them and would like to maintain them, I'm not against
that.

We could also do some cleanup in the linux ones, as there are likely a few architectures
that could be considered obsolete.  However, in the case of Linux, somebody motivated
could always build a toolchain and sysroot themselves.  For reference, here are the
architectures not currently supported in the upstream Linux kernel:

- bfin (removed in 4.16)
- cris (and crisv32 I guess) (removed in 4.17)
- m32r (removed in 4.16)
- tic6x (I don't think it was ever supported upstream.  Looking at this [1], there doesn't
  seem to be development since ~2012)
- tile (removed in 4.16)

In my opinion, we should remove the corresponding GDBserver ports, unless somebody shows
interest for them.  For reference, Linux 4.16 has been released more than two years ago.

About Windows support for ARM, I don't really know about it.  I think that our port
was targeting Windows CE [2], which can probably be considered obsolete.  However,
Windows 10 supposedly runs on ARM [3], so it might be relevant to keep it?  I don't really
know if the current GDBserver code would help for that or not.  In doubt, I won't propose
to remove it.

Please let me know what you think.

Also, does anybody know if you deprecation/removal process is written somewhere?  I know
we discussed it in the past, but I can't find it.

Simon

[1] http://www.linux-c6x.org/wiki/index.php/Releases
[2] https://en.wikipedia.org/wiki/Windows_Embedded_Compact
[3] https://docs.microsoft.com/en-us/windows/arm/