fixes #25007; implements setLenUninit for refc (#25022)

fixes #25007

```nim
proc setLengthSeqUninit(s: PGenericSeq, typ: PNimType, newLen: int, isTrivial: bool): PGenericSeq {.
    compilerRtl.} =
```

In this added function, only the line `zeroMem(dataPointer(result,
elemAlign, elemSize, newLen), (result.len-%newLen) *% elemSize)` is
removed from `proc setLengthSeqV2` when enlarging a sequence.

JS and VM versions simply use `setLen`.

tests/stdlib/tsystem.nim

@@ -236,5 +236,12 @@ proc bar2() =
     doAssert cstring(nil) <= cstring(nil)
     doAssert cstring("") <= cstring("")
 
+  var x = @[10, 20]
+  x.setLenUninit(5)
+  x[4] = 50
+  doAssert x[4] == 50
+  x.setLenUninit(1)
+  doAssert x == @[10]
+
 static: bar2()
 bar2()