[llvm] [sanitizer][msan] fix AArch64 vararg support for KMSAN (PR #70660)

[Evgenii Stepanov via llvm-commits]

eugenis wrote:

To avoid similar problems in the future, please make it so that userspace and kernel variants of this function have the same type requirements by either:
* adding a pointer cast to the kernel variant
* adding a type assert to the userspace variant

https://github.com/llvm/llvm-project/pull/70660