libstdc++: Avoid illegal argument to verbose in dg-test callback, continued

This is a follow-up to commit 697b94cf
"libstdc++: Avoid illegal argument to verbose in dg-test callback".
I'm confirming the original problem, but on one system, it's not
resolved by this change, because instead we get:

    extra_tool_flags are:
    ERROR: tcl error sourcing [...]/libstdc++-v3/testsuite/libstdc++-dg/conformance.exp.
    ERROR: usage: send [args] string
        while executing
    "send_log "$message\n""
        (procedure "verbose" line 48)
        invoked from within
    "verbose -log -- $extra_tool_flags"
        (procedure "libstdc++-dg-test" line 45)
        invoked from within
    "${tool}-dg-test $prog [lindex ${dg-do-what} 0] "$tool_flags ${dg-extra-tool-flags}""
        (procedure "saved-dg-test" line 115)
        invoked from within
    [...]

That's Ubuntu's dejagnu 1.5-3ubuntu1 being so old that it doesn't include
DejaGnu commit 57c22601afe43d2c2b8819df4f2ecacb034516fd "Protect from leading
dash in message".  (I suppose that's what'd make this work, but have not
verified.)

	libstdc++-v3/
	* testsuite/lib/libstdc++.exp: Avoid illegal argument to verbose,
	continued.