perf tools: Add 'build-test' make target

perf tools: Add 'build-test' make target

Currently various build test can be performed using a Makefile named
tests/make, so one needs to remember and specify it with -f option on
command line.

Add the 'build-test' target in the main Makefile as a shortcut.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1389837173-3632-1-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>