testsuite, Darwin: Handle undefined symbols in pr97172-2 testcase.

Darwin's linker defaults to complaining about missing symbols, leading
to a spurious fail of this testcase.  We can work around this by
allowing undefined symbols to be considered as dynamically looked-up.

gcc/testsuite/ChangeLog:

	* gcc.dg/pr97172-2.c: Allow undefined symbols at link-time.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>