Allow emcc build

Emcc generates Wasm but not a native executable, making the generated
bin2c non-executable. As a result, the device tree blob cannot be built
into a C application. To resolve this, fall back to a native compiler
gcc or clang to build bin2c.

Additionally, ensuring CC is set to gcc, clang, or emcc. Otherwise,
trigger a build error early.

Related: #534

Author: ChinYikMing

mk/system.mk

@@ -12,9 +12,17 @@ $(BUILD_DTB): $(DEV_SRC)/minimal.dts
 	$(VECHO) " DTC\t$@\n"
 	$(Q)$(CC) -nostdinc -E -P -x assembler-with-cpp -undef $(CFLAGS_dt) $^ | $(DTC) - > $@
 
+# Assume the system has either GCC or Clang
+NATIVE_CC := $(shell which gcc || which clang)
+
 BIN_TO_C := $(OUT)/bin2c
 $(BIN_TO_C): tools/bin2c.c
+# emcc generates wasm but not executable, so fallback to use GCC or Clang
+ifeq ("$(CC_IS_EMCC)", "1")
+	$(Q)$(NATIVE_CC) -Wall -o $@ $^
+else
 	$(Q)$(CC) -Wall -o $@ $^
+endif
 
 BUILD_DTB2C := src/minimal_dtb.h
 $(BUILD_DTB2C): $(BIN_TO_C) $(BUILD_DTB)

mk/toolchain.mk

@@ -52,6 +52,10 @@ else ifneq ($(shell $(CC) --version | grep "Free Software Foundation"),)
      CC_IS_GCC := 1
 endif
 
+ifeq ("$(CC_IS_CLANG)$(CC_IS_GCC)$(CC_IS_EMCC)", "")
+$(error "Only supported GCC/Clang/Emcc")
+endif
+
 CFLAGS_NO_CET :=
 processor := $(shell uname -m)
 ifeq ($(processor),$(filter $(processor),i386 x86_64))