examples/wasm/wasm_sample: revert prebuild WASM

Running `make` in the wasm example modifies the `hello_prebuild.wasm`
example, making it easy to sneak in unwanted changes. This reverts such
an instance and modifies the Makefile to only recreate/update
`hello_prebuild.wasm` with:

    make hello_prebuild.wasm

examples/wasm/wasm_sample/Makefile

@@ -104,6 +104,8 @@ COMPILE_FLAGS := -Wall \
 %.wat: %.wasm Makefile
 	wasm2wat -o $@ $<
 
+hello.wasm:
+
 ifeq ($(COPY_HELLO),YES)
 hello.wasm: hello_prebuild.wasm
 	@echo "!! NO wasm-ld(-VERSION) found  !!"