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

[Dmitry Vasilyev via llvm-commits]

slydiman wrote:

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

Thanks.
Pre-commit failed on Linux x64 because of flang.

I will complete tests with LLVM_ENABLE_RPMALLOC before commit. No rush.

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