perf, MIPS: Support cross compiling of tools/perf for MIPS

Changes:
 v4: Fix the cosmetic issue of redundant dot-ops
 v3: Change rmb() to use SYNC
 v2: Include mips unistd.h and define rmb()/cpu_relax() in tools/perf/perf.h

Signed-off-by: Deng-Cheng Zhu <dengcheng.zhu@gmail.com>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
Cc: David Daney <ddaney@caviumnetworks.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>