Makefile: fix default SYNO_ARCH in Makefile

It was broken with the move to dist in 32e0ba5 which doesn't accept
amd64 anymore.

Updates #cleanup

Change-Id: Iaaaba2d73c6a09a226934fe8e5c18b16731ee7a6
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

Makefile

@@ -1,5 +1,5 @@
 IMAGE_REPO ?= tailscale/tailscale
-SYNO_ARCH ?= "amd64"
+SYNO_ARCH ?= "x86_64"
 SYNO_DSM ?= "7"
 TAGS ?= "latest"