[libc++][z/OS] Correct a definition of __native_vector_size (#91995)

Fix `std/ranges/range.adaptors/range.lazy.split/general.pass.cpp` which
started failing on z/OS after this
[commit](https://github.com/llvm/llvm-project/commit/985c1a44f8d49e0af).

This test case is passing on other platforms such as AIX. This is
because the `__ALTIVEC__` macro is defined and `__mismatch` under
`_LIBCPP_VECTORIZE_ALGORITHMS` guard is compiled out. However, on z/OS
`_LIBCPP_VECTORIZE_ALGORITHMS` is defined. Analyzing the algorithm of
`__mismatch` shows that the culprit is the definition of
`__native_vector_size` which was defined wrongly as 1. This PR corrects
the definition of `__native_vector_size` and fixes the affected test.