use ARCH instead of BUILD for Makefile

Signed-off-by: Wang Chen <wangchen20@iscas.ac.cn>

Makefile

@@ -2,48 +2,53 @@
 # SHOULD NOT be submitted to upstream!!!
 
 # some default values
-BUILD ?= riscv64
+ARCH ?= riscv64
 CLANG ?= /aosp/common/wangchen/llvm-build/Release+Asserts
 T ?= chrome_public_apk
 
-# make clean [BUILD=<xxx>] [T=<xxx>]
-# <xxx> e.g. 
-# buildtools/third_party/libc++:libc++
-# chrome_public_apk
+# make clean [ARCH=<xxx>] [T=<xxx>]
+# Value of ARCH:
+# - riscv64
+# - arm64
+# - ...
+# Value of T:
+# e.g.
+# - buildtools/third_party/libc++:libc++
+# - chrome_public_apk
 .PHONY: clean
 clean:
 	@echo Start cleaning target: "$(T)"
-	autoninja -C out/$(BUILD) -t clean $(T)
+	autoninja -C out/$(ARCH) -t clean $(T)
 	@echo Done!
 
-# make ninja [BUILD=<xxx>] [T=<xxx>] [V=1]
+# make ninja [ARCH=<xxx>] [T=<xxx>] [V=1]
 # build target and create log file
 # To get more verbose log, add argument "V=1"
 .PHONY: ninja
 ninja:
 	@echo Start building target: "$(T)"
 	-cp ./log ./log.bak
 ifeq ($(V),1)
-	autoninja -d keeprsp -v -C out/$(BUILD) $(T) 2>&1 | tee ./log
+	autoninja -d keeprsp -v -C out/$(ARCH) $(T) 2>&1 | tee ./log
 else
-	autoninja -d keeprsp -C out/$(BUILD) $(T) 2>&1 | tee ./log
+	autoninja -d keeprsp -C out/$(ARCH) $(T) 2>&1 | tee ./log
 endif
 	@echo Done!
 
-# make disclean [BUILD=<xxx>]
+# make disclean [ARCH=<xxx>]
 .PHONY: distclean
 distclean:
-	rm -rf out/$(BUILD)
+	rm -rf out/$(ARCH)
 
-# make gn [BUILD=<xxx>] [CLANG=<xxx>]
+# make gn [ARCH=<xxx>] [CLANG=<xxx>]
 # FIXME:
 # "clang_use_chrome_plugins=false" may not be required when we are using
 # clang for Chrome built by ourselves.
 .PHONY: gn
 gn:
-	gn gen out/$(BUILD) --args="\
+	gn gen out/$(ARCH) --args="\
 	target_os=\"android\" \
-	target_cpu=\"riscv64\" \
+	target_cpu=\"$(ARCH)\" \
 	skip_secondary_abi_for_cq=true \
 	android_static_analysis=\"off\" \
 	clang_base_path=\"$(CLANG)\" \