btrace: Enable event tracing on Linux for Intel PT.

btrace: Enable event tracing on Linux for Intel PT.

Event tracing allows GDB to show information about interesting asynchronous
events when tracing with Intel PT.  Subsequent patches will add support for
displaying each type of event.

Enabling event-tracing unconditionally would result in rather noisy output, as
breakpoints themselves result in interrupt events.  Which is why this patch adds
a set/show command to allow the user to enable/disable event-tracing before
starting a recording. The event-tracing setting has no effect on an already
active recording.  The default setting is off.   As event tracing will use the
auxiliary infrastructure added by ptwrite, the user can still disable printing
events, even when event-tracing was enabled, by using the /a switch for the
record instruction-history/function-call-history commands.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Markus Metzger <markus.t.metzger@intel.com>