[powerpc] deactivate readability-identifier-naming.cpp test on powerpc64le

The test case clang-tidy/readability-identifier-naming.cpp segfaults on
powerpc64 little endian (starting with r288563) when a bootstrap build/test
is done.  To get the buildbot running again deactivate the test.
When the issue is resolved reactivate it.

llvm-svn: 289581