testsuite: 32 bit AIX 2 byte wchar

32 bit AIX supports 2 byte wchar.  The wchar-multi1.C testcase assumes
4 byte wchar.  Update the testcase to require 4 byte wchar.

gcc/testsuite/ChangeLog:
	* g++.dg/cpp23/wchar-multi1.C: Require 4 byte wchar_t.

Signed-off-by: David Edelsohn <dje.gcc@gmail.com>