Use numeric defines for ARCH.
diff --git a/cli/build.go b/cli/build.go
index df8002e..43d4612 100644
--- a/cli/build.go
+++ b/cli/build.go
@@ -147,7 +147,7 @@
 		cflags += " -DTEST"
 	}
 
-	cflags += " -DARCH=" + t.Arch
+	cflags += " -DARCH_" + t.Arch
 
 	// If a non-BSP package is being built, add the BSP's C flags to the list.
 	// The BSP's compiler flags get exported to all packages.