Avoid technically UB type punning when determining endianness

include/internal/catch_tostring.cpp

@@ -38,13 +38,11 @@ namespace Detail {
             enum Arch { Big, Little };
 
             static Arch which() {
-                union _{
-                    int asInt;
-                    char asChar[sizeof (int)];
-                } u;
-
-                u.asInt = 1;
-                return ( u.asChar[sizeof(int)-1] == 1 ) ? Big : Little;
+                int one = 1;
+                // If the lowest byte we read is non-zero, we can assume
+                // that little endian format is used.
+                auto value = *reinterpret_cast<char*>(&one);
+                return value ? Little : Big;
             }
         };
     }