perf build: Add llvm/clang/cxx make tests into FEATURE_TESTS_EXTRA

perf build: Add llvm/clang/cxx make tests into FEATURE_TESTS_EXTRA

So we can see the status when we build perf, like:

  $ make LIBCLANGLLVM=1 VF=1
  ...                           cxx: [ on  ]
  ...                          llvm: [ on  ]
  ...                  llvm-version: [ on  ]
  ...                         clang: [ on  ]

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180307155020.32613-18-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>