Reduce libcore parallelism for host

Temporary fix to get the tests green again.

Test: run-libcore-tests.py --mode=host
Change-Id: Ib52db885dcdc7e41d9ddeeea389e3e0ec7a343c6
diff --git a/tools/run-libcore-tests.py b/tools/run-libcore-tests.py
index 7a980c2..afbc62f 100755
--- a/tools/run-libcore-tests.py
+++ b/tools/run-libcore-tests.py
@@ -226,7 +226,13 @@
       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))