wasm-unknown: make sure the os package can be imported

See: https://github.com/tinygo-org/tinygo/issues/4314

The os package isn't particularly useful on wasm-unknown, but just like
on baremetal systems it's imported by a lot of packages so it should at
least be possible to import this package.

src/os/removeall_noat.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build !baremetal && !js && !wasip1
+//go:build !baremetal && !js && !wasip1 && !wasm_unknown
 
 package os