Move artifact target under emcc build condition

Normal 'make' or 'make all' does not require downloading prebuilt ELF
executables from the rv32emu-prebuilt repo. However, 'deps_emcc' always
depends on the 'artifact' target, even when not using emcc, causing 'make'
or 'make all' to always download prebuilt ELF executables. To prevent this,
the 'artifact' target is moved under the emcc build condition.

Related: #491

Author: ChinYikMing

mk/wasm.mk

@@ -1,5 +1,5 @@
 CFLAGS_emcc ?=
-deps_emcc := artifact
+deps_emcc :=
 ASSETS := assets
 WEB_HTML_RESOURCES := $(ASSETS)/html
 WEB_JS_RESOURCES := $(ASSETS)/js
@@ -42,7 +42,7 @@ $(OUT)/elf_list.js: tools/gen-elf-list-js.py
 	$(Q)tools/gen-elf-list-js.py > $@
 
 # used to download all dependencies of elf executable and bundle into single wasm
-deps_emcc += $(OUT)/elf_list.js $(DOOM_DATA) $(QUAKE_DATA) $(TIMIDITY_DATA)
+deps_emcc += artifact $(OUT)/elf_list.js $(DOOM_DATA) $(QUAKE_DATA) $(TIMIDITY_DATA)
 
 # check browser MAJOR version if supports TCO
 CHROME_MAJOR :=