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/runtime/nonhosted.go

@@ -1,4 +1,4 @@
-//go:build baremetal || js
+//go:build baremetal || js || wasm_unknown
 
 package runtime