Fix WebAssembly build requiring DTB dependencies

The emscripten build for SYSTEM mode embeds build/minimal.dtb via
--embed-file, but after commit c6638da which fixed DTB compilation
to be conditional, the DTB was not being built for emscripten builds.

Add $(BUILD_DTB) and $(BUILD_DTB2C) to deps_emcc when ENABLE_SYSTEM=1
AND ENABLE_ELF_LOADER=0 AND CC_IS_EMCC=1 to ensure the DTB is built
before emscripten attempts to embed it.

The triple-nested condition is necessary because:
- DTB compilation requires CFLAGS_dt macros (MEM_START, INITRD_START, etc.)
- These macros are only defined when SYSTEM=1 AND ELF_LOADER=0
- Emscripten SYSTEM builds use the default ELF_LOADER=0
- CC_IS_EMCC guard prevents DTB from being built during `make distclean ENABLE_SYSTEM=1`
  when CC is not emcc (e.g., macOS workflow running distclean between builds)

Fixes emscripten build errors:
  error: build/minimal.dtb@/minimal.dtb does not exist
  Error: <stdin>:12.31-32 syntax error (undefined macros)

Fixes macOS workflow regression where `make distclean ENABLE_SYSTEM=1`
incorrectly triggered DTB compilation when CC was not emcc.

Author: jserv

mk/wasm.mk

@@ -65,6 +65,17 @@ $(OUT)/elf_list.js: artifact tools/gen-elf-list-js.py
 # used to download all dependencies of elf executable and bundle into single wasm
 deps_emcc += artifact $(OUT)/elf_list.js $(DOOM_DATA) $(QUAKE_DATA) $(TIMIDITY_DATA)
 
+# For SYSTEM mode, emscripten needs the DTB file to embed
+# DTB is only built when SYSTEM=1 and ELF_LOADER=0
+# Only add when actually using emcc, not when cleaning
+ifeq ("$(CC_IS_EMCC)", "1")
+ifeq ($(call has, SYSTEM), 1)
+ifeq ($(call has, ELF_LOADER), 0)
+deps_emcc += $(BUILD_DTB) $(BUILD_DTB2C)
+endif
+endif
+endif
+
 # check browser version if supports TCO
 CHROME_MAJOR :=
 CHROME_MAJOR_VERSION_CHECK_CMD :=