As noted in PR 96817 this new test fails if the library is built without
futexes. That's expected of course, but we might as well fail more
obviously than a deadlock that eventually times out.

libstdc++-v3/ChangeLog:

	* testsuite/18_support/96817.cc: Fail fail if the library is
	configured to not use futexes.

Tested powerpc64le-linux. Committed to trunk.

I've just realised the changelog above should say "Fail fast", I'll
fix that in the ChangeLog tomorrow.