Do not XFAIL test/tools/llvm-cov tests on powerpc64le

All tests in test/tools/llvm-cov fail on big-endian targets and are
supposed to be XFAILed there.  However, including "powerpc64" in the
XFAIL line is now incorrect, since that matches both powerpc64- and
powerpc64le- targets, and the tests pass on the latter.

Update the XFAIL lines to use powerpc64- instead (like mips64-).