Use std::hexfloat more. (#6203)

Previously, we were only using std::hexfloat on Windows on VS2013
and later.

Since std::hexfloat is part of C++11 and we require C++11 to build
the Z3 library, this should be supported everywhere.

src/util/mpf.cpp

@@ -1630,11 +1630,7 @@ std::string mpf_manager::to_string_hexfloat(mpf const & x) {
                                     std::ios_base::showpoint | std::ios_base::showpos);
     ss.setf(ff);
     ss.precision(13);
-#if defined(_WIN32) && _MSC_VER >= 1800
     ss << std::hexfloat << to_double(x);
-#else
-    ss << std::hex << (*reinterpret_cast<const unsigned long long *>(&(x)));
-#endif
     return ss.str();
 }