perf build: Move feature cleanup under tools/build

Arnaldo reported issue for following build command:

  $ rm -rf /tmp/krava; mkdir /tmp/krava; make O=/tmp/krava clean
    CLEAN    config
  /bin/sh: line 0: cd: /tmp/krava/feature/: No such file or directory
  ../../scripts/Makefile.include:17: *** output directory "/tmp/krava/feature/" does not exist.  Stop.
  make[1]: *** [Makefile.perf:1010: config-clean] Error 2
  make: *** [Makefile:90: clean] Error 2

The problem is that now that we include scripts/Makefile.include
in feature's Makefile (which is fine and needed), we need to ensure
the OUTPUT directory exists, before executing (out of tree) clean
command.

Removing the feature's cleanup from perf Makefile and fixing
feature's cleanup under build Makefile, so it now checks that
there's existing OUTPUT directory before calling the clean.

Fixes: 211a741c

 ("tools: Factor Clang, LLC and LLVM utils definitions")
Reported-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Jiri Olsa <jolsa@k...>