build: Linux arm64 support

* protoc zip fetching
* Makefile:
    * GOOS and GOARCH
    * run vet on all targets

Note: freebsd/arm64 is apparently not supported

fixes #180
refs #181

build.installprotoc.bash

@@ -0,0 +1,25 @@
+#!/usr/bin/env bash
+set -euo pipefail
+set -x
+
+MACH=$(uname -m)
+MACH="${MACH/aarch64/aarch_64}"
+
+VERSION=3.6.1
+FILENAME=protoc-"$VERSION"-linux-"$MACH".zip
+
+if [ -e "$FILENAME" ]; then
+    echo "$FILENAME" already exists 1>&2
+    exit 1
+fi
+
+wget https://github.com/protocolbuffers/protobuf/releases/download/v"$VERSION"/"$FILENAME"
+
+stat "$FILENAME"
+
+sha256sum -c --ignore-missing <<EOF
+6003de742ea3fcf703cfec1cd4a3380fd143081a2eb0e559065563496af27807  protoc-3.6.1-linux-x86_64.zip
+af8e5aaaf39ddec62ec8dd2be1b8d9602c6da66564883a16393ade5f71170922  protoc-3.6.1-linux-aarch_64.zip
+EOF
+
+unzip -d /usr "$FILENAME"