server/etcdmain: add build support for Apple M1

This has been additionally verified by running the tests locally as a
basic smoke test. GitHub Actions doesn't provide MacOS M1 (arm64) yet,
so there's no good way to automate testing.

Ran `TMPDIR=/tmp make test` locally. The `TMPDIR` bit is needed so
there's no really long path used that breaks Unix socket setup in one of
the tests.

scripts/build-binary

@@ -79,6 +79,10 @@ function main {
       TARGET_ARCHS+=("s390x")
     fi
 
+    if [ ${GOOS} == "darwin" ]; then
+      TARGET_ARCHS+=("arm64")
+    fi
+
     for TARGET_ARCH in "${TARGET_ARCHS[@]}"; do
       export GOARCH=${TARGET_ARCH}