Revert^3 "Reduce libcore parallelism for host"

This reverts commit f2f924222a4ec748bdeab45c4de7d88ae477c594.

Reason for revert: It made the tests green after all.
(I still consider this as only short-term work-around)

Change-Id: Id027c5ef437abf7b2018374ded5c9529a6bb4867
diff --git a/tools/run-libcore-tests.py b/tools/run-libcore-tests.py
index 3ebee09..ba3c090 100755
--- a/tools/run-libcore-tests.py
+++ b/tools/run-libcore-tests.py
@@ -218,7 +218,13 @@
       raise AssertionError(f"Missing {jar}. Run buildbot-build.sh first.")
 
   if not args.jobs:
-    args.jobs = get_target_cpu_count() if args.mode == "device" else multiprocessing.cpu_count()
+    if args.mode == "device":
+      args.jobs = get_target_cpu_count()
+    else:
+      args.jobs = multiprocessing.cpu_count()
+      if args.gcstress:
+        # TODO: Investigate and fix the underlying issues.
+        args.jobs = args.jobs // 2
 
   def run_test(test_name):
     cmd = " ".join(get_vogar_command(test_name))