spawn.signal_stress: reap spawned children on exit

Test: adb shell \
  /data/nativetest64/bionic-unit-tests-static/bionic-unit-tests-static \
  --gtest_filter=spawn.signal_stress
Bug: b/77824306
Change-Id: I970d1a4d0f8ac9621e676f32f838fc2f638d9750
(cherry picked from commit 344969c8)