[libcxx] Refactoring SIMD function names in PSTL CPU backend (#69029)

This PR addresses a smaller detail discussed in the code review for
https://github.com/llvm/llvm-project/pull/66968. Currently, some
functions in the `libc++` PSTL CPU backend have been appended with a
digit to indicate the number of input iterator arguments. However, there
is no need to change the name for each version as overloading can be
used instead. This PR will make the naming more consistent in the the
CPU and the proposed OpenMP backend.