Add mips64 to arches

This is the architecture being used by EdgeRouter-12 for example.

Makefile

@@ -3,7 +3,7 @@ BIN := $(shell basename $$PWD)
 COMMIT := $(shell git describe --dirty --always)
 TAG := $(shell git describe --tags --dirty || echo latest)
 LDFLAGS := "-s -w -X github.com/coredns/coredns/coremain.GitCommit=$(COMMIT)"
-ARCHS := "linux/amd64,linux/arm64"
+ARCHS := "linux/amd64,linux/arm64,linux/mips64"
 
 # Where to push the docker image.
 REGISTRY ?= quay.io/oriedge