Added 'sim6502' and 'sim65C02' targets.

The targets allow to run cc65 programs in the sim65 exection
einvironment. As there are no "real" i/o facilities there's no
need for header files. Paravirtualized entry points are mapped
to $FFF0 ff. There's a large cc65 progam area from $0200-$FFEF.

The binary format includes a one-byte header indicating the required
execution environment: The value 0 means 6502 and the value 1
means 65C02. The load adress for the binary is fixed to $0200.

Note: Running sim65C02 programs currently doesn't work bcause
sim65 doesn't actually implement 65C02 opcodes.

libsrc/sim6502/crt0.s

@@ -0,0 +1,32 @@
+;
+; Oliver Schmidt, 2013-05-16
+;
+; Startup code for cc65 (sim6502 version)
+;
+
+        .export         _exit
+        .export         __STARTUP__ : absolute = 1      ; Mark as startup
+        .import         zerobss, callmain
+        .import         initlib, donelib
+        .import         exit
+        .import         __RAM_START__, __RAM_SIZE__     ; Linker generated
+        .import         __STACKSIZE__                   ; Linker generated
+
+        .include        "zeropage.inc"
+
+        .segment        "STARTUP"
+
+        cld
+        ldx     #$FF
+        txs
+        lda     #<(__RAM_START__ + __RAM_SIZE__ + __STACKSIZE__)
+        ldx     #>(__RAM_START__ + __RAM_SIZE__ + __STACKSIZE__)
+        sta     sp
+        stx     sp+1
+        jsr     zerobss
+        jsr     initlib
+        jsr     callmain
+_exit:  pha
+        jsr     donelib
+        pla
+        jsr     exit