[gcc r13-6783] libstdc++: Fix simd test compilation with Clang

[Matthias Kretz <mkretz@gcc.gnu.org>]

https://gcc.gnu.org/g:25db59017f2e216a76c8a0fe4814568135f19f9a

commit r13-6783-g25db59017f2e216a76c8a0fe4814568135f19f9a
Author: Matthias Kretz <m.kretz@gsi.de>
Date:   Thu Feb 23 14:45:07 2023 +0100

    libstdc++: Fix simd test compilation with Clang
    
    Signed-off-by: Matthias Kretz <m.kretz@gsi.de>
    
    libstdc++-v3/ChangeLog:
    
            * testsuite/experimental/simd/tests/operators.cc: Clang doesn't
            define __GCC_IEC_559. Use __STDC_IEC_559__ instead.

Diff:
---
 libstdc++-v3/testsuite/experimental/simd/tests/operators.cc | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/testsuite/experimental/simd/tests/operators.cc b/libstdc++-v3/testsuite/experimental/simd/tests/operators.cc
index c4d91fa5d2b..72af7061c73 100644
--- a/libstdc++-v3/testsuite/experimental/simd/tests/operators.cc
+++ b/libstdc++-v3/testsuite/experimental/simd/tests/operators.cc
@@ -211,7 +211,14 @@ template <typename V>
     }
 
     // divides
-    constexpr bool is_iec559 = __GCC_IEC_559 >= 2;
+    constexpr bool is_iec559 =
+#ifdef __GCC_IEC_559
+      __GCC_IEC_559 >= 2;
+#elif defined __STDC_IEC_559__
+      true;
+#else
+      false;
+#endif
     if constexpr (std::is_floating_point_v<T> && !is_iec559)
       { // avoid testing subnormals and expect minor deltas for non-IEC559 float
 	V x = 2;