[powerpc] deactivate flakey tests on powerpc64le

These test cases occassionally fail when run on powerpc64le:

ignore_lib1.cc
ignore_lib5.cc
TestCases/Posix/current_allocated_bytes.cc
rtl/TsanRtlTest/Posix.ThreadLocalAccesses
TestCases/Posix/coverage-fork-direct.cc

The failures cause false problem reports to be sent to developers whose
code had nothing to do with the failures.  Reactivate them when the real
problems are fixed.

This could also be related to the same problems as with the tests
ThreadedOneSizeMallocStressTest, ThreadedMallocStressTest, ManyThreadsTest,
and several others that do not run reliably on powerpc.

llvm-svn: 301798