Add process startup code

start.68k

@@ -0,0 +1,19 @@
+  ifnd __FREESTANDING
+  xref main
+  include include/syscalls.i
+  section .text, text
+  public _start
+  _start:
+  move.l #__libstd_start_stack_top, a7
+  ; set up a permanent stack
+  move.l #2, d0
+  move.l #$2, d1
+  jsr syscall_vmem_map_free
+  adda.l #(2*4096), a0
+  move.l a0, a7
+  jsr main
+  jsr syscall_exit
+  section .bss,bss
+__libstd_start_stack: ds.w 16
+__libstd_start_stack_top:
+  endif