Update CPC port to SDCC 4.2.0

This version of SDCC introduces a new calling convention where some
values can be passed in registers. Adjust the assembler code
accordingly, making it smaller and faster.

Some things are not working quirte right yet:
- The routine to clear a rectangle isn't working correctly
- The keyboard input also does not seem to work completely
diff --git a/contiki-cpc/arch/contiki.lnk b/contiki-cpc/arch/contiki.lnk
index ebf8a1e..49240a3 100644
--- a/contiki-cpc/arch/contiki.lnk
+++ b/contiki-cpc/arch/contiki.lnk
@@ -1,5 +1,5 @@
 -imwn contiki
--k /usr/local/share/sdcc/lib/z80
+-k /system/data/sdcc/lib/z80
 -b _CODE=0x0100
 -l z80
 ./arch/crt0.o