tools build: Fix feature Makefile issues with 'O='

When building perf binaries outside the source tree with 'make O=<dir>',
the auto-detected features get re-tested for every build, which is
unnecessary and inconsistent with the behavior seen when building
directly in the source tree.

Another issue is that 'make O=<dir> clean' doesn't remove the feature
files from the object tree.

Fix these problems by looking for the binaries in the $(OUTPUT)
directory.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/113bd01530e9761778c60a75a96c65fc59860f68.1450193761.git.jpoimboe@redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>