Add CPC putchar to the linker file.

The default implementation uses RST 8, which is not going to work for us.
diff --git a/contiki-cpc/arch/contiki.lnk b/contiki-cpc/arch/contiki.lnk
index 29d5b16..42d2f85 100644
--- a/contiki-cpc/arch/contiki.lnk
+++ b/contiki-cpc/arch/contiki.lnk
@@ -25,4 +25,5 @@
 ../contiki/apps/processes-dsc.o
 ./arch/main.o
 ./arch/arch.o
+./arch/putchar.o
 -e