perf: Add attr->precise support to raw event parsing

Minimal userspace interface to the new 'precise' events flag.

Can be used like "perf top -e r00c0p" which will use PEBS to sample
retired instructions.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@infradead.org>
Cc: paulus@samba.org
Cc: eranian@google.com
Cc: robert.richter@amd.com
Cc: fweisbec@gmail.com
LKML-Reference: <20100304140100.468665803@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>