rt-tests: allow building subset of tests with non-NPTL toolchains

rt-tests: allow building subset of tests with non-NPTL toolchains

Some architectures are still stuck with non-NPTL toolchains.
These are for example ARC, Blackfin, Xtensa etc.

Still rt-tests are very good benchmarks and it would be good to enable use of
at least selected (those that will be built) tests on those architectures.

This change makes it possible to only build subset of tests that don't require
NPTL calls.

Following tests will be built with non-NPTL toolchain:
 * signaltest
 * ptsematest
 * sigwaittest
 * svsematest
 * sendme
 * hackbench

Still it's required to have a toolchain with threads support because most of
mentioned tests use threads.

03-fix-non-nptl-buil.patch was submitted upstream:
https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg762958.html
so as soon as it is accepted with the next version bump this patch should be
removed.

[Thomas: fix the rt-tests.mk test on NPTL to use positive logic.]

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>