rv: Add runtime reactors interface

A runtime monitor can cause a reaction to the detection of an
exception on the model's execution. By default, the monitors have
tracing reactions, printing the monitor output via tracepoints.
But other reactions can be added (on-demand) via this interface.

The user interface resembles the kernel tracing interface and
presents these files:

"available_reactors"
  - Reading shows the available reactors, one per line.

   For example:
     # cat available_reactors
     nop
     panic
     printk

 "reacting_on"
   - It is an on/off general switch for reactors, disabling
   all reactions.

 "monitors/MONITOR/reactors"
   - List available reactors, with the select reaction for the given
   MONITOR inside []. The default one is the nop (no operation)
   reactor.
   - Writing the name of a reactor enables it to the given
   MONITOR.

   For example:
     # cat monitors/wip/reactors
     [nop]
     panic
     printk
     # echo panic > monitors/wip/reactors
     # cat monitors/wip/reactors
     nop
     [panic]
     printk

Link: https://lkml.kernel.org/r/1794eb994637457bdeaa6bad0b8263d2f7eece0c.1659052063.git.bristot@kernel.org

Cc: Wim Van Sebroeck <[email protected]>
Cc: Guenter Roeck <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Marco Elver <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Gabriele Paoloni <[email protected]>
Cc: Juri Lelli <[email protected]>
Cc: Clark Williams <[email protected]>
Cc: Tao Zhou <[email protected]>
Cc: Randy Dunlap <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Daniel Bristot de Oliveira <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

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