Fix WebAssembly build requiring DTB dependencies

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

Root cause: The $(BIN) target (linking stage) uses CFLAGS_emcc which
contains --embed-file build/minimal.dtb, but had no dependency on the
DTB files. Adding DTB to deps_emcc only affects .o file compilation,
not the final linking step.

Solution:
1. Add $(BUILD_DTB) and $(BUILD_DTB2C) to deps_emcc for .o compilation
2. Add explicit $(BIN) dependency on DTB files for the linking stage
3. Guard both with: CC_IS_EMCC=1 AND SYSTEM=1 AND ELF_LOADER=0

The 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 with different compilers).

Fixes emscripten build errors:
  error: build/minimal.dtb@/minimal.dtb does not exist
  emcc: error: 'file_packager ... --embed build/minimal.dtb ...' failed

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 +65,19 @@ $(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
+ifeq ("$(CC_IS_EMCC)", "1")
+ifeq ($(call has, SYSTEM), 1)
+ifeq ($(call has, ELF_LOADER), 0)
+# Add DTB as dependency for both compilation and linking
+deps_emcc += $(BUILD_DTB) $(BUILD_DTB2C)
+# Make the final binary depend on DTB files (for linking stage)
+$(BIN): $(BUILD_DTB) $(BUILD_DTB2C)
+endif
+endif
+endif
+
 # check browser version if supports TCO
 CHROME_MAJOR :=
 CHROME_MAJOR_VERSION_CHECK_CMD :=