[sancov] enabling tests on ppc and s390
The reason they should have failed: absent sancov.
In this test (vs the trace_pc_guard test) sancov is not
yet called (not implemented).
// Tests trace pc guard coverage collection.
//
// REQUIRES: has_sancovcc,stable-runtime
-// XFAIL: tsan,darwin,powerpc64,s390x
+// XFAIL: tsan,darwin
+// TODO: this will fail on powerpc64,s390x once sancov will be called.
//
// RUN: DIR=%t_workdir
// RUN: rm -rf $DIR
