[TSAN] Disable clone_setns test case on PPC64 RHEL 7.9 Targets

The compler-rt test case tsan/Linux/clone_setns.cpp fails on
PowerPC64 RHEL 7.9 targets.
Unshare fails with errno code EINVAL.
It is unclear why this happens specifically on RHEL 7.9 and no other
operating system like Ubuntu 18 or RHEL 8.4 for example.
This patch uses marcos to disable the test case for ppc64 rhel7.9
because there are no XFAIL directives to target rhel 7.9 specifically.

Reviewed By: dvyukov

Differential Revision: https://reviews.llvm.org/D130086