perf trace: Use $PERF_EXEC_PATH in canned report scripts

Set $PERF_EXEC_PATH before starting the record and report scripts, and
make them use it where necessary.

Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1286723403.2955.205.camel@localhost>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>