run gcext test single threaded

Revert "run gcext test single threaded"

This reverts commit 4f8668c.

Update gcext.c

Author: IanButterworth

test/gcext/gcext.c

@@ -600,6 +600,12 @@ int main()
     jl_gc_set_cb_notify_external_alloc(alloc_bigval, 1);
     jl_gc_set_cb_notify_external_free(free_bigval, 1);
 
+    // single threaded mode
+    jl_options.nthreadpools = 1;
+    jl_options.nthreads = 1;
+    int16_t ntpp[] = {jl_options.nthreads};
+    jl_options.nthreads_per_pool = ntpp;
+
     jl_init();
     if (jl_gc_enable_conservative_gc_support() < 0)
         abort();