perf tools: Fix parallel build including 'clean' target

perf tools: Fix parallel build including 'clean' target

Do not parallelize 'clean' with other targets, figure out if it is
present and do it first, then the other targets.

Noticed with:

  tools/perf> make -j24 clean all

   LD       arch/libperf-in.o
   LD       plugin_xen-in.o
 arch//libperf-in.o: file not recognized: File truncated
 make[3]: *** [arch/libperf-in.o] Error 1
 make[2]: *** [arch] Error 2
 make[2]: *** Waiting for unfinished jobs....
   AR       libapi.a

Reported-and-Tested-by: Arnaldo Carvalho de Melo <[email protected]>
Signed-off-by: Jiri Olsa <[email protected]>
Acked-by: Wang Nan <[email protected]>
Link: http://lkml.kernel.org/n/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>