Added library reference em_libref to EMD interface.

Occasionally dynamically drivers suffer from not being to refer to
content in the C library. Therefore I added a mechanism to allow
a C library for a certian target to define a symbol that will be
handed over to dynamic drivers for that target. Then the drivers
can use their refernce to that symbol to access content in the C
library.

libsrc/apple2/emd/a2.auxmem.s

@@ -20,16 +20,20 @@
         .byte   $65, $6d, $64           ; "emd"
         .byte   EMD_API_VERSION         ; EM API version number
 
-; Jump table.
+; Library reference
 
-        .word   INSTALL
-        .word   DEINSTALL
-        .word   PAGECOUNT
-        .word   MAP
-        .word   USE
-        .word   COMMIT
-        .word   COPYFROM
-        .word   COPYTO
+        .addr   $0000
+
+; Jump table
+
+        .addr   INSTALL
+        .addr   UNINSTALL
+        .addr   PAGECOUNT
+        .addr   MAP
+        .addr   USE
+        .addr   COMMIT
+        .addr   COPYFROM
+        .addr   COPYTO
 
 ; ------------------------------------------------------------------------
 ; Constants
@@ -69,11 +73,11 @@ INSTALL:
 ;       rts
 
 ; ------------------------------------------------------------------------
-; DEINSTALL routine. Is called before the driver is removed from memory.
+; UNINSTALL routine. Is called before the driver is removed from memory.
 ; Can do cleanup or whatever. Must not return anything.
 ;
 
-DEINSTALL:
+UNINSTALL:
         rts