configs: Fix x86-gapbs-benchmarks.py example

With https://gem5-review.googlesource.com/c/public/gem5/+/64791 we
updated the configs/example/gem5_library to utilize the `m5.simulate`
module. The GAPBS benchmark example uses the "WORKBEGIN" and "WORKEND"
exit events to specify the ROI. The patch incorrectly assumed an "EXIT"
exit event were used.

As such, the
"test-gem5-library-example-x86-gapbs-benchmarks-ALL-x86_64-opt-MESI_Two_Level"
test was not properly running, causing the Nightly test to fail:
https://jenkins.gem5.org/job/nightly/444. This patch fixes this error.

Change-Id: I207fe3563c8d9c59bcb79428fe62d2d2bbccd013
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/66514
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>

configs/example/gem5_library/x86-gapbs-benchmarks.py

@@ -211,7 +211,7 @@ board.set_kernel_disk_workload(
 )
 
 
-def handle_exit():
+def handle_workbegin():
     print("Done booting Linux")
     print("Resetting stats at the start of ROI!")
     m5.stats.reset()
@@ -219,6 +219,9 @@ def handle_exit():
     start_tick = m5.curTick()
     processor.switch()
     yield False  # E.g., continue the simulation.
+
+
+def handle_workend():
     print("Dump stats at the end of the ROI!")
     m5.stats.dump()
     yield True  # Stop the simulation. We're done.
@@ -227,7 +230,8 @@ def handle_exit():
 simulator = Simulator(
     board=board,
     on_exit_event={
-        ExitEvent.EXIT: handle_exit(),
+        ExitEvent.WORKBEGIN: handle_workbegin(),
+        ExitEvent.WORKEND: handle_workend(),
     },
 )