fixes regression #20746; remove string copies for ORC booted compiler (#20776)

* fixes #20746; remove string copies for ORC booted compiler

* add a test case

* use `cursor` thanks to @beef331

* for old compilers

* change file extension

* change test cases

compiler/vm.nim

@@ -32,6 +32,8 @@ const
 when hasFFI:
   import evalffi
 
+when not defined(nimHasCursor):
+  {.pragma: cursor.}
 
 proc stackTraceAux(c: PCtx; x: PStackFrame; pc: int; recursionLimit=100) =
   if x != nil:
@@ -787,7 +789,7 @@ proc rawExecute(c: PCtx, start: int, tos: PStackFrame): TFullReg =
     of opcLdStrIdx:
       decodeBC(rkInt)
       let idx = regs[rc].intVal.int
-      let s = regs[rb].node.strVal
+      let s {.cursor.} = regs[rb].node.strVal
       if idx <% s.len:
         regs[ra].intVal = s[idx].ord
       else:

tests/vm/t20746.nim

@@ -0,0 +1,13 @@
+discard """
+  timeout: 10
+  joinable: false
+  output: "fine"
+"""
+
+func addString(): string =
+  let x = newString(1000000)
+  for i in 0..<1000000:
+    discard x[i]
+
+const translationTable = addString()
+echo "fine"