c++: Update get_cxx_dialect_name for cxx29

Update get_cxx_dialect_name to handle cxx29 added by

commit 0c5c6249a2e9a75d2e7d1c6c434487c8dc077130
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Jun 16 22:24:57 2026 +0200

    c++, libcpp: Add -std=c++2[9d] and -std=gnu++2[9d] options

	PR c++/125851
	* name-lookup.cc (get_cxx_dialect_name): Handle cxx29.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>