testsuite: Fix weak_undefined handling on Darwin

The patch that introduced the weak_undefined effective-target keyword
and corresponding dg-add-options support

commit 378ec7b8
Author: Alexandre Oliva <oliva@adacore.com>
Date:   Thu Mar 23 00:45:05 2023 -0300

    [testsuite] test for weak_undefined support and add options

badly broke the affected tests on macOS like so:

ERROR: gcc.dg/addr_equal-1.c: unknown dg option: 89 for " dg-add-options 5 weak_undefined "
ERROR: gcc.dg/addr_equal-1.c: unknown dg option: 89 for " dg-add-options 5 weak_undefined "

add_options_for_weak_undefined tries to call an non-existant proc "89".
Even after fixing this by escaping the brackets, two tests still failed to
link since they lacked the corresponding calls do dg-add-options
weak_undefined.

Tested on x86_64-apple-darwin20.6.0 and i386-pc-solaris2.11.

2023-03-27  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	gcc/testsuite:
	* lib/target-supports.exp (add_options_for_weak_undefined): Escape
	brackets.
	* gcc.dg/visibility-22.c: Add weak_undefined options.

	libgomp:
	* testsuite/libgomp.oacc-c-c++-common/routine-nohost-2.c: Add
	weak_undefined options.