perf tools: Use __weak definition from linux/compiler.h

Replace __attribute__((weak)) with __weak definition

Signed-off-by: Rui Teng <rui.teng@linux.vnet.ibm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1469671557-2256-2-git-send-email-rui.teng@linux.vnet.ibm.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>