C++, testsuite: Adjust pure-virtual1.C options for Darwin.

C++, testsuite: Adjust pure-virtual1.C options for Darwin.

Darwin's static linker expects, by default, that all linkage will
be satisfied (even for weak items).  This allows the two-level
namespace to work even for weak cases.

However, there's an option to make it behave in a similar manner
to ELF.  This is needed for the pure-virtual1.C testcase to pass
on Darwin.

gcc/testsuite/ChangeLog:

	* g++.dg/abi/pure-virtual1.C: Add dynamic_lookup option
	for Darwin.