[llvm] Fixed cmake config in case of LLVM_ENABLE_RPMALLOC=ON and non MSVC compiler (PR #139770)

[Mészáros Gergely via llvm-commits]

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

LGTM. Assuming you've verified that this works, because I did not. Also please wait for pre-commit to finish before landing.

https://github.com/llvm/llvm-project/pull/139770