testsuite: tsan: add fallback overload for pthread_cond_clockwait

LTS GNU/Linux distros from 2018, still in use, don't have
pthread_cond_clockwait.  There's no trivial way to detect it so as to
make the test conditional, but there's an easy enough way to silence
the fail due to lack of the function in libc, and that has nothing to
do with the false positive that this is testing against.


for  gcc/testsuite/ChangeLog

	* g++.dg/tsan/pthread_cond_clockwait.C: Add fallback overload.