[gdb/build] Workaround gcc PR113599

[gdb/build] Workaround gcc PR113599

Since gcc commit d3f48f68227 ("c++: non-dependent .* operand folding
[PR112427]"), with gdb we run into PR gcc/113599 [1], a wrong-code bug, as
reported in PR build/31281.

Work around this by flipping inherit order:
...
-class thread_info : public refcounted_object,
-		    public intrusive_list_node<thread_info>
+class thread_info : public intrusive_list_node<thread_info>,
+		    public refcounted_object
...

An argument could be made that this isn't necessary, because this occurred in
an unreleased gcc version.

However, I think it could be useful when bisecting gcc for other problems in
building gdb.  Having this workaround means the bisect won't reintroduce the
problem.  Furthermore, the workaround is harmless.

Tested on Fedora rawhide x86_64.

Approved-By: Tom Tromey <tom@tromey.com>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31281

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113599