Move core:runtime to base:runtime; keep alias around

base/runtime/os_specific_darwin.odin

@@ -0,0 +1,12 @@
+//+build darwin
+package runtime
+
+import "core:intrinsics"
+
+_os_write :: proc "contextless" (data: []byte) -> (int, _OS_Errno) {
+	ret := intrinsics.syscall(0x2000004, 1, uintptr(raw_data(data)), uintptr(len(data)))
+	if ret < 0 {
+		return 0, _OS_Errno(-ret)
+	}
+	return int(ret), 0
+}