[libc] [llvm] feat: implement template meta muli (PR #138654)

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PiJoules wrote:

The implementation looks ok, but could you add tests and the relevant build changes so we can ensure it works properly? You can follow what the other fixed point patches do and replace the names appropriately (https://github.com/llvm/llvm-project/pull/133005 for example).

https://github.com/llvm/llvm-project/pull/138654