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

core/iwasm/libraries/wasi-nn/src/utils/logger.h

@@ -21,7 +21,7 @@
     3 -> err
     4 -> NO LOGS
 */
-#define NN_LOG_LEVEL 0
+#define NN_LOG_LEVEL 2
 #endif
 
 // Definition of the levels