Fix possible deadlock when waiting for mutex_threads

Usually when we wait for getting a lock on a mutex, we mark the waitingthread
as blocked so another thread may steal our GC work. This isn't a good idea for
the instance's mutex_threads though as the GC also acquires a lock on that
mutex and we can end up in a dead lock. Since all places that use this lock
only hold it briefly, marking the thread as blocked isn't really that
beneficial anyway.

The first place that got this wrong was MVM_proc_fork and I copied that mistake
to MVM_spesh_stats_cleanup. Both are now fixed.

Fixes GH#4462

Author: niner

src/io/procops.c

@@ -1251,9 +1251,9 @@ MVMint64 MVM_proc_fork(MVMThreadContext *tc) {
     /* Allow MVM_io_eventloop_start to restart the thread if necessary */
     instance->event_loop_thread = NULL;
 
-    MVM_gc_mark_thread_blocked(tc);
+    /* Do not mark thread blocked as the GC also tries to acquire
+     * mutex_threads and it's help only briefly by all holders anyway */
     uv_mutex_lock(&instance->mutex_threads);
-    MVM_gc_mark_thread_unblocked(tc);
 
     /* Check if we are single threaded and if true, fork() */
     if (MVM_thread_cleanup_threads_list(tc, &instance->threads) == 1) {

src/spesh/stats.c

@@ -670,9 +670,9 @@ void MVM_spesh_stats_cleanup(MVMThreadContext *tc, MVMObject *check_frames) {
                      * consideration. */
                 }
                 else if (tc->instance->spesh_stats_version - ss->last_update > MVM_SPESH_STATS_MAX_AGE) {
-                    MVM_gc_mark_thread_blocked(tc);
+                    /* Do not mark thread blocked as the GC also tries to acquire
+                     * mutex_threads and it's help only briefly by all holders anyway */
                     uv_mutex_lock(&tc->instance->mutex_threads);
-                    MVM_gc_mark_thread_unblocked(tc);
 
                     MVMThread *current = tc->instance->threads;
                     int found = 0;