Apple: Add pthread implementation of Thread class

This allows Apple platforms to override the default stack size of
a thread in the WorkerThreadPool, which is 512KiB by default.

This must be increased, as SPIRV-Cross, used by the Metal driver, can
use deeply nested stacks, as can debug builds.

tests/core/templates/test_rid.h

@@ -140,7 +140,7 @@ TEST_CASE("[RID_Owner] Thread safety") {
 			uint32_t target = (p_step / 2 + 1) * threads.size();
 			sync[buf_idx].fetch_add(1, std::memory_order_relaxed);
 			do {
-				std::this_thread::yield();
+				Thread::yield();
 			} while (sync[buf_idx].load(std::memory_order_relaxed) != target);
 		}