libstdc++: Fix testcase by using terminate handler

libstdc++: Fix testcase by using terminate handler

This test was supposed to verify that when __libc_single_threaded is
available we successfully detect recursive static initialization even
when linked to libpthread. But I forgot to that when recursive init is
detected, we terminate, and so the test fails.

This adds a terminate handler that exits cleanly, so the test passes
when recursive init is detected.

libstdc++-v3/ChangeLog:

	* testsuite/18_support/96817.cc: Use terminate handler that
	calls _Exit(0).