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

[Tobias Hieta via llvm-commits]

tru wrote:

> I would keep it for now for testing other allocators. I still find mimalloc being a very good contender and I hope it'll be eventually integrated into Windows through the application manifest maybe. Also, it'd be good if someone with SCUDO experience would make it work with `LLVM_INTEGRATED_CRT_ALLOC`.

If we keep it around - maybe we need to update the documentation to point out that using RPMALLOC is the most supported way and that this option is only for testing other allocators? Seems like we want people to use the vendored version of rpmalloc most of the time.

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