cmd/composer: move currentArch helper to common package

The helper function might be useful also in different parts of the project.

internal/common/helpers.go

@@ -0,0 +1,17 @@
+package common
+
+import "runtime"
+
+func CurrentArch() string {
+	if runtime.GOARCH == "amd64" {
+		return "x86_64"
+	} else if runtime.GOARCH == "arm64" {
+		return "aarch64"
+	} else if runtime.GOARCH == "ppc64le" {
+		return "ppc64le"
+	} else if runtime.GOARCH == "s390x" {
+		return "s390x"
+	} else {
+		panic("unsupported architecture")
+	}
+}