x86, bts: use trace_clock_global() for timestamps

Rename the bts_struct timestamp field to event.

Use trace_clock_global() for time measurement.

Reported-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>
Cc: roland@redhat.com
Cc: eranian@googlemail.com
Cc: oleg@redhat.com
Cc: juan.villacis@intel.com
Cc: ak@linux.jf.intel.com
LKML-Reference: <20090403144553.773216000@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>