Fix the "register native with iwasm" stuff for macOS (#1558)

- core/shared/platform/darwin/platform_internal.h: macOS has dlopen
- samples/native-lib/README.md: Mention macOS

samples/native-lib/README.md

@@ -45,11 +45,20 @@ will be generated.
 
 ## Run workload
 
+### Linux
+
 ```bash
 cd build
 ./iwasm --native-lib=libtest_add.so --native-lib=libtest_sqrt.so wasm-app/test.wasm
 ```
 
+### macOS
+
+```bash
+cd build
+./iwasm --native-lib=libtest_add.dylib --native-lib=libtest_sqrt.dylib wasm-app/test.wasm
+```
+
 The output is:
 
 ```bash