sim: prefix --debug-flags Event logs with the flag name

sim: prefix --debug-flags Event logs with the flag name

Sample output of FmtFlag,ExecAll,Event:

   0: Event: Event_70: generic event rescheduled @ 18446744073709551615
   0: ExecEnable: system.cpu: A0 T0 : @asm_main_after_prologue
   0: Event: AtomicSimpleCPU tick.wrapped_function_event
 500: ExecEnable: system.cpu: A0 T0 : @asm_main_after_prologue+4
 500: Event: AtomicSimpleCPU tick.wrapped_function_event
1000: ExecEnable: system.cpu: A0 T0 : @asm_main_after_prologue+8
1000: Event: AtomicSimpleCPU tick.wrapped_function_event
1500: ExecEnable: system.cpu: A0 T0 : @asm_main_after_prologue+12
1500: Event: AtomicSimpleCPU tick.wrapped_function_event

Change-Id: I7f252b57d7778a15a3dda40d909bdb4425557a40
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22009
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>