[llvm] [Support] Vendor rpmalloc in-tree and use it for the Windows 64-bit release (PR #91862)

[via llvm-commits]

https://github.com/zmodem approved this pull request.

The `s/DLIBXML2_INCLUDE_DIRS/DLIBXML2_INCLUDE_DIR/` change is still there after the latest commit. Tackling that in a separate PR sounds good to me.

Besides that, this lgtm

https://github.com/llvm/llvm-project/pull/91862