Added support for ARCH=s390x (#532)

Author: guirish

Makefile

@@ -26,6 +26,10 @@ else ifeq ($(GOARCH), arm64)
 ARCH_TAG_PREFIX=$(GOARCH)
 FILE_ARCH=ARM aarch64
 DOCKERFILE_SED_EXPR?=
+else ifeq ($(GOARCH), s390x)
+ARCH_TAG_PREFIX=$(GOARCH)
+FILE_ARCH=IBM S/390
+DOCKERFILE_SED_EXPR?=
 else
 DOCKERFILE_SED_EXPR?=
 FILE_ARCH=x86-64