perf build-test: Honour JOBS to override detection of number of cores

perf build-test: Honour JOBS to override detection of number of cores

When one does:

  $ make -C tools/perf build-test

The makefile in tools/perf/tests/ will, just like the main one, detect
how many cores are in the system and use it with -j.

Sometimes we may need to override that, for instance, when using
icecream or distcc to use multiple machines in the build process, then
we need to, as with the main makefile, use:

  $ make JOBS=N -C tools/perf build-test

Fix the tests makefile to honour that.

Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lore.kernel.org/lkml/20200330130301.GA31702@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>