libstdc++: Enable simple invocation of runtest in testsuite

2020-05-21  Matthias Kretz  <kretz@kde.org>

	* testsuite/Makefile.am: Remove dup target_triplet and set tool,
	allowing runtest to work without arguments.
	* testsuite/Makefile.in: Regenerate.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>