[libc] Fix a constexpr violation from b4e5529

In msan mode this calls __msan_unpoison, which isn't constexpr.

Author: d0k

libc/src/__support/FPUtil/except_value_utils.h

@@ -102,7 +102,7 @@ template <typename T, size_t N> struct ExceptValues {
 // Helper functions to set results for exceptional cases.
 template <typename T> LIBC_INLINE T round_result_slightly_down(T value_rn) {
   volatile T tmp = value_rn;
-  constexpr T MIN_NORMAL = FPBits<T>::min_normal().get_val();
+  const T MIN_NORMAL = FPBits<T>::min_normal().get_val();
   tmp = tmp - MIN_NORMAL;
   return tmp;
 }