[gdb/symtab] Work around PR gas/29517, dwarf2 case

In commit 1d45d909 ("[gdb/symtab] Work around PR gas/29517") we added a
workaround for PR gas/29517.

The problem is present in gas version 2.39, and fixed in 2.40, so the
workaround is only active for gas version == 2.39.

However, the problem in gas is only fixed for dwarf version >= 3, which
supports DW_TAG_unspecified_type.

Fix this by also activating the workaround for dwarf version == 2.

Tested on x86_64-linux.

Approved-by: Kevin Buettner <kevinb@redhat.com>

PR symtab/31689
https://sourceware.org/bugzilla/show_bug.cgi?id=31689