[NFC][libcxx] Check def of __LONG_DOUBLE_128__

[daltenty]

Summary

Follow on to https://reviews.llvm.org/D108352, also check for the definition of __LONG_DOUBLE_128__ in this test before using it (because AIX doesn't enable it by default).

libcxx/test/std/numerics/numeric.ops/numeric.ops.midpoint/midpoint.float.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
-// XFAIL: LIBCXX-AIX-FIXME
-
 // <numeric>
 
 // template <class _Float>
 // _Tp midpoint(_Float __a, _Float __b) noexcept
 //
 
 #include <numeric>
 #include <cassert>

 
 //  Check two values "close to each other"
     T d1 = T(3.14);
     T d0 = std::nextafter(d1, T(2));
     T d2 = std::nextafter(d1, T(5));
     assert(d0 < d1);  // sanity checking
     assert(d1 < d2);  // sanity checking
 
-#if defined(__PPC__) && __LONG_DOUBLE_128__ && !(defined(__LONG_DOUBLE_IEEE128__) && __LONG_DOUBLE_IEEE128__)
+#if defined(__PPC__) && (defined(__LONG_DOUBLE_128__) && __LONG_DOUBLE_128__) &&                                       \
+    !(defined(__LONG_DOUBLE_IEEE128__) && __LONG_DOUBLE_IEEE128__)
 //  For 128 bit long double implemented as 2 doubles on PowerPC,
 //  nextafterl() of libm gives imprecise results which fails the
 //  midpoint() tests below. So skip the test for this case.
     if constexpr (sizeof(T) != 16)
 #endif
     {
   //  Since there's nothing in between, the midpoint has to be one or the other
     T res;