perf utils: Readlink /proc/self/exe to find the perf binary

Simplification: it is easier to open /proc/self/exe than /proc/$pid/exe.

Signed-off-by: Tommi Rantala <tommi.t.rantala@nokia.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20170322130624.21881-7-tommi.t.rantala@nokia.com


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