syscall: disable TestAllThreadsSyscall on linux-ppc64
syscall: disable TestAllThreadsSyscall on linux-ppc64

For some reason, currently unknown, this test case fails exclusively
on the linux-ppc64 platform. Until such time as it can be made to
work, we'll disable this test case on that platform.

The same issue causes TestSetuidEtc to fail too, so disable that
on this platform.

Updates #42178

Change-Id: Idd3f6c2ee9f2fba2eb8ce4de69de7f316858bb15
Reviewed-on: https://go-review.googlesource.com/c/go/+/264719
Trust: Emmanuel Odeke <277f8eaa8c7dfbc55c17bf5e30561c1a9c45c569@gmail.com>
Run-TryBot: Emmanuel Odeke <277f8eaa8c7dfbc55c17bf5e30561c1a9c45c569@gmail.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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