[libc++] Fix one case in saturate_cast.pass.cpp for 64-bit on z/OS (#86724)

On z/OS int128 is disabled causing one of the cases in
`saturate_cast.pass.cpp` to fail. The failure is only in 64-bit mode.
In this case `the std::numeric_limits<long long int>::max()` is within
`std::numeric_limits<unsigned long int>::min()`
and `std::numeric_limits<unsigned long int>::max()` therefore,
saturate_cast<unsigned long int>( sBigMax) == LONG_MAX and not ULONG_MAX
as original test.

In 32-bit, `saturate_cast<unsigned long int>( sBigMax) == ULONG_MAX`
like on other platforms where int128 is enabled.

This PR is required to pass this test case on z/OS and possibly on other
platforms where int128 is not supported/enabled.

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Co-authored-by: Sean Perry <perry@ca.ibm.com>