C++ conversion status update

Hi all,

[Project page here: https://sourceware.org/gdb/wiki/cxx-conversion]

Good news, we've reached a significant milestone on the C++ conversion -- on
a few important hosts, GDB now builds cleanly as a C++ program with no
hacks.  These are at least:

 - Aarch64 GNU/Linux
 - ARM GNU/Linux
 - x86 GNU/Linux
 - x86_64 GNU/Linux
 - x86_64 Mingw-w64

In addition, the testsuite shows no regressions in C++ mode, compared to C mode.

Up until recently, we were still using g++'s -fpermissive switch as a shortcut.
That allowed deferring fixing a huge number boring cast and enum conversion issues.
Most of those issues are now fixed, and since the hosts above cover most of
the common, non-host-specific code, we changed the build system to no longer
use g++'s -fpermissive switch, and at the same time, made C++ mode default to
build with -Werror too, just like C mode.  Big thanks goes to Simon Marchi,
Yao Qi and others for all the help.

There's still a lot of work ahead though.  Until all supported hosts can build
with a C++ compiler, we'll have to keep supporting building GDB as a C program
as well.  It's not sufficient to say "builds cleanly on GNU/Linux", because
each architecture+OS combination (ARM GNU/Linux, x86 Windows, etc.) has it's
own set of native debugger support files (the *-nat.c files) that are not built
on any other host.

The plan I propose next is to default to building in C++ mode on hosts
that are known to build cleanly in C++ mode.  Or rather, change the default
to be C++ mode, _except_ for hosts/ports that haven't been converted yet.
Then, as ports get converted, one by one they'll be removed from
the "can't do C++ yet" list.  That gives us a defined stopping point -- when
the list becomes empty, and after a reasonable period, we can decide to remove
support for building in C mode and start making use of C++ features.

The question I have is which hosts (Architecture + OS combination) people
care about that still need C++ conversion work and thus should be on
that list?

This is where anyone can help!

I've created the "users/palves/cxx-conversion" branch (on sourceware.org)
where I committed a patch that implements the list idea.  I filled in
a set of hosts that I _thought_ wouldn't build, but I don't really know
for sure.

So I'd like to invite people to try to build master on their favorite
host with --enable-build-with-cxx, and report back the result.

Thanks,
Pedro Alves