Make wasi-nn backends as separated shared libraries (#3509)

- All files under *core/iwasm/libraries/wasi-nn* are compiled as shared libraries
- *wasi-nn.c* is shared between backends
- Every backend has a separated shared library
- If wasi-nn feature is enabled, iwasm will depend on shared library libiwasm.so
  instead of linking static library libvmlib.a

product-mini/platforms/posix/main.c

@@ -295,7 +295,8 @@ load_native_lib(const char *name)
     /* open the native library */
     if (!(lib->handle = dlopen(name, RTLD_NOW | RTLD_GLOBAL))
         && !(lib->handle = dlopen(name, RTLD_LAZY))) {
-        LOG_WARNING("warning: failed to load native library %s", name);
+        LOG_WARNING("warning: failed to load native library %s. %s", name,
+                    dlerror());
         goto fail;
     }