build: make rclone build with wasm

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

lib/mmap/mmap_unix.go

@@ -1,7 +1,7 @@
 // Package mmap implements a large block memory allocator using
 // anonymous memory maps.
 
-// +build !plan9,!windows
+// +build !plan9,!windows,!js
 
 package mmap
 

lib/mmap/mmap_unsupported.go

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