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

[Mattias Jansson via llvm-commits]

mjansson wrote:

As author of rpmalloc I consent to it being contributed under the LLVM licensing terms, both the new Apache w/ LLVM license and the legacy NCSA license.

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