Make VL_LOCK_SPINS configurable

It's unlikely one value fits all use case, so making VL_LOCK_SPINS
configurable at model build time.

For testing, we reduce the value as we expect high contention.

include/verilated.h

@@ -164,7 +164,9 @@ inline constexpr size_t roundUpToMultipleOf(size_t value) {
 // Return current thread ID (or 0), not super fast, cache if needed
 extern uint32_t VL_THREAD_ID() VL_MT_SAFE;
 
+#ifndef VL_LOCK_SPINS
 #define VL_LOCK_SPINS 50000  /// Number of times to spin for a mutex before yielding
+#endif
 
 /// Mutex, wrapped to allow -fthread_safety checks
 class VL_CAPABILITY("mutex") VerilatedMutex final {

test_regress/Makefile_obj

@@ -39,6 +39,9 @@ CPPFLAGS += $(CPPFLAGS_DRIVER)
 CPPFLAGS += $(CPPFLAGS_DRIVER2)
 CPPFLAGS += $(CPPFLAGS_ADD)
 
+# Reduce spin count for faster testing
+CPPFLAGS += -DVL_LOCK_SPINS=10000
+
 ifeq ($(CFG_WITH_LONGTESTS),yes)
   ifeq ($(DRIVER_STD),newest)
     CPPFLAGS += $(CFG_CXXFLAGS_STD)