Add Mac M1 support

.gitignore

@@ -1 +1,2 @@
 .build
+*.swp

Makefile

@@ -62,6 +62,8 @@ ifeq ($(arch1),x86_64)
 arch2=amd64
 else ifeq ($(arch1),aarch64)
 arch2=arm64
+else ifeq ($(arch1),arm64)
+arch2=arm64
 else
 $(error unsupported ARCH: $(arch1))
 endif
@@ -102,8 +104,8 @@ endif
 # protoc
 #############################################################################
 
-protoc_version = 3.14.0
-ifeq ($(arch1),aarch64)
+protoc_version = 3.20.1
+ifeq ($(arch2),arm64)
 protoc_url = https://github.com/protocolbuffers/protobuf/releases/download/v$(protoc_version)/protoc-$(protoc_version)-$(os2)-aarch_64.zip
 else
 protoc_url = https://github.com/protocolbuffers/protobuf/releases/download/v$(protoc_version)/protoc-$(protoc_version)-$(os2)-$(arch1).zip