fixes #25007; implements setLenUninit for refc

[ringabout]

fixes #25007

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.

[github-actions]

Thanks for your hard work on this PR!
The lines below are statistics of the Nim compiler built from 611b8bb

Hint: mm: orc; opt: speed; options: -d:release
182867 lines; 8.794s; 659.902MiB peakmem