add rp6502 target

libsrc/rp6502/crt0.s

@@ -0,0 +1,54 @@
+; Copyright (c) 2023 Rumbledethumps
+;
+; SPDX-License-Identifier: Zlib
+; SPDX-License-Identifier: BSD-3-Clause
+; SPDX-License-Identifier: Unlicense
+
+; Boilerplate crt0.s
+
+.export _init, _exit
+.import _main
+
+.export __STARTUP__ : absolute = 1
+.import __RAM_START__, __RAM_SIZE__
+
+.import copydata, zerobss, initlib, donelib
+
+.include "rp6502.inc"
+.include "zeropage.inc"
+
+.segment  "STARTUP"
+
+; Essential 6502 startup the CPU doesn't do
+_init:
+    ldx #$FF
+    txs
+    cld
+
+; Set cc65 argument stack pointer
+    lda #<(__RAM_START__ + __RAM_SIZE__)
+    sta sp
+    lda #>(__RAM_START__ + __RAM_SIZE__)
+    sta sp+1
+
+; Initialize memory storage
+    jsr zerobss   ; Clear BSS segment
+    jsr copydata  ; Initialize DATA segment
+    jsr initlib   ; Run constructors
+
+; Call main()
+    jsr _main
+
+; Back from main() also the _exit entry
+; Stack the exit value in case destructors call OS
+_exit:
+    phx
+    pha
+    jsr donelib  ; Run destructors
+    pla
+    sta RIA_A
+    plx
+    stx RIA_X
+    lda #$FF     ; exit()
+    sta RIA_OP
+    stp