Add thread support code

Provides very simple ConcurrentQueue and ThreadPool classes that build even when threading is disabled.

Also provides a `DeferredLogs` class that captures log output to be replayed on the main thread
later.

kernel/threading.cc

@@ -0,0 +1,45 @@
+#include "kernel/yosys_common.h"
+#include "kernel/threading.h"
+
+YOSYS_NAMESPACE_BEGIN
+
+void DeferredLogs::flush()
+{
+	for (auto &m : logs)
+		if (m.error)
+			YOSYS_NAMESPACE_PREFIX log_error("%s", m.text.c_str());
+		else
+			YOSYS_NAMESPACE_PREFIX log("%s", m.text.c_str());
+}
+
+int ThreadPool::pool_size(int reserved_cores, int max_threads)
+{
+#ifdef YOSYS_ENABLE_THREADS
+	int num_threads = std::min<int>(std::thread::hardware_concurrency() - reserved_cores, max_threads);
+        return std::max(0, num_threads);
+#else
+        return 0;
+#endif
+}
+
+ThreadPool::ThreadPool(int pool_size, std::function<void(int)> b)
+	: body(std::move(b))
+{
+#ifdef YOSYS_ENABLE_THREADS
+        threads.reserve(pool_size);
+        for (int i = 0; i < pool_size; i++)
+                threads.emplace_back([i, this]{ body(i); });
+#else
+        log_assert(pool_size == 0);
+#endif
+}
+
+ThreadPool::~ThreadPool()
+{
+#ifdef YOSYS_ENABLE_THREADS
+	for (auto &t : threads)
+		t.join();
+#endif
+}
+
+YOSYS_NAMESPACE_END