tsan: disable signal_sync2.cpp test on powerpc64

Fails 1 out of 10 runs on powerpc bots:
https://lab.llvm.org/buildbot/#/builders/121/builds/13391

Reviewed By: nemanjai

Differential Revision: https://reviews.llvm.org/D114426

GitOrigin-RevId: d75ed9864acb15d1543e839fe4627c9b1d867c22
1 file changed