[libc] [clang] [llvm] [Libc] Give more functions restrict qualifiers (PR #78061)

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AtariDreams wrote:

> LLVM changes look unrelated, it was originally copied from OpenBSD it seems. But it's not a major issue.

I did that for consistency. If needed I could split into other PRs

https://github.com/llvm/llvm-project/pull/78061