rename "sp" to "spc", avoid conflict with 4510 opcodes

libsrc/runtime/sub.s

@@ -6,7 +6,7 @@
 
         .export         tossuba0, tossubax
         .import         addysp1
-        .importzp       sp
+        .importzp       spc
 
         .macpack        cpu
 
@@ -18,17 +18,17 @@ tossubax:
         sec
         eor     #$FF
 .if (.cpu .bitand CPU_ISET_65SC02)
-        adc     (sp)
+        adc     (spc)
         ldy     #1
 .else
         ldy     #0
-        adc     (sp),y          ; Subtract low byte
+        adc     (spc),y          ; Subtract low byte
         iny
 .endif
         pha                     ; Save high byte
         txa
         eor     #$FF
-        adc     (sp),y          ; Subtract high byte
+        adc     (spc),y          ; Subtract high byte
         tax                     ; High byte into X
         pla                     ; Restore low byte
         jmp     addysp1         ; drop TOS