build: make rclone build with wasm

Needed to drop
- azureblob backend
- cache backend
- qingstor backend
- cachestats command
- ncdu command

lib/mmap/mmap_unsupported.go

@@ -1,6 +1,6 @@
 // Fallback Alloc and Free for unsupported OSes
 
-// +build plan9
+// +build plan9 js
 
 package mmap