tools/perf/build: Fix non-canonical directory names in O=

This was a long-standing bug, relative pathnames like O=dir did not fully
work in the build system:

    $ make O=localdir clean

    SUBDIR Documentation
    ../../scripts/Makefile.include:3: *** O=localdir does not exist.  Stop.
    make[1]: *** [clean] Error 2
    make: *** [clean] Error 2

Fix this by canonizing the directory before passing it to Makefile.perf.

Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/n/tip-hchMp1hozn9tqgswWcooxcru@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>