[compiler-rt] [rtsan] Fix va_args handling in open functions (PR #108291)

[Chris Apple via llvm-commits]

cjappl wrote:

@kubamracek  - could you weigh in on if this is necessary or not? I see you did the original work adding va_args to the tsan `open` interceptor.

Is there anything unsafe about the original way we were doing this?

https://github.com/llvm/llvm-project/pull/108291