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 <acme@redhat.com>
Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-kb0qs29zbz7hxn32mc5zbsoz@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>