[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.

---------

Co-authored-by: Sean Perry <[email protected]>

Author: zibi2

libcxx/test/std/numerics/numeric.ops/numeric.ops.sat/saturate_cast.pass.cpp

@@ -329,7 +329,7 @@ constexpr bool test() {
   { [[maybe_unused]] std::same_as<unsigned long int> decltype(auto) _ = std::saturate_cast<unsigned long int>(sBigMax); }
   assert(std::saturate_cast<unsigned long int>(  sBigMin) == 0UL);       // saturated
   assert(std::saturate_cast<unsigned long int>(    sZero) == 0UL);
-  assert(std::saturate_cast<unsigned long int>(  sBigMax) == ULONG_MAX); // saturated
+  assert(std::saturate_cast<unsigned long int>(  sBigMax) == (sizeof(UIntT) > sizeof(unsigned long int) ? ULONG_MAX : LONG_MAX)); // saturated depending on underlying types
 
   { [[maybe_unused]] std::same_as<unsigned long int> decltype(auto) _ = std::saturate_cast<unsigned long int>(uBigMax); }
   assert(std::saturate_cast<unsigned long int>(    uZero) == 0UL);