Import GNU Readline 8.2

This imports readline 8.2 patch 13.

This time around I thought I would try to document the process.

First I have a checkout of the upstream readline repository.  I make a
local branch there, based on the previous upstream import.  In this
case that was readline 8.1; see gdb commit b4f26d541aa.

Then, I apply all readline changes from the gdb repository since the
previous readline import.  In this case that is up to commit
3dee0baea2e in the gdb repo.

After this, I "git merge" from the relevant upstream commit.  In the
past I feel like I used a tag, but readline is managed very strangely
and I didn't see a tag.  So I just used the patch 13 commit, aka
commit 037d85f1 upstream.

Then I fixed all the merge conflicts.  Re-running autoconf requires a
symlink from '../../config' into the gdb tree, due to the local
m4_include addition.  It's possible other hacks like this are
required, I don't remember how I set things up in the past.

After this, I did a build + test of gdb.  I also did a mingw
cross-hosted build, because that's caused build failures in past
imports.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32265
Reviewed-by: Sam James <sam@gentoo.org>