[libc++][math.h] Remove some code duplication

[mstorsjo]

No specific opinion on this from my side - if it works for all clang-cl and mingw configs in CI, it's most probably fine with me. But CI seems to have not run correctly - it seems to have run before the parent patch was set up, so it failed to apply the patch. Rerunning it, now that the parent relationship is set up already, should hopefully make it apply it correctly and run the tests.

[ldionne]

LGTM if CI is happy. This seems to basically removes copy-pasted code which isn't relevant anymore now that we're using Clang builtins.

[philnik]

Rebased

libcxx/include/math.h

 #include <limits>
 #include <stdlib.h>
 #include <type_traits>
 
 // signbit
 
 #    ifdef signbit
 #      undef signbit
+#    endif // signbit
 
 template <class _A1>
 inline _LIBCPP_HIDE_FROM_ABI typename std::enable_if<std::is_floating_point<_A1>::value, bool>::type
 signbit(_A1 __x) _NOEXCEPT {
   return __builtin_signbit(__x);
 }
 
 template <class _A1>
 inline _LIBCPP_HIDE_FROM_ABI
     typename std::enable_if< std::is_integral<_A1>::value && std::is_signed<_A1>::value, bool>::type
     signbit(_A1 __x) _NOEXCEPT {
   return __x < 0;
 }
 
 template <class _A1>
 inline _LIBCPP_HIDE_FROM_ABI
     typename std::enable_if< std::is_integral<_A1>::value && !std::is_signed<_A1>::value, bool>::type
     signbit(_A1) _NOEXCEPT {
   return false;
 }
 
-#    elif defined(_LIBCPP_MSVCRT)
-
-template <typename _A1>
-inline _LIBCPP_HIDE_FROM_ABI typename std::enable_if<std::is_floating_point<_A1>::value, bool>::type
-signbit(_A1 __x) _NOEXCEPT {
-  return ::signbit(__x);
-}
-
-template <class _A1>
-inline _LIBCPP_HIDE_FROM_ABI
-    typename std::enable_if< std::is_integral<_A1>::value && std::is_signed<_A1>::value, bool>::type
-    signbit(_A1 __x) _NOEXCEPT {
-  return __x < 0;
-}
-
-template <class _A1>
-inline _LIBCPP_HIDE_FROM_ABI
-    typename std::enable_if< std::is_integral<_A1>::value && !std::is_signed<_A1>::value, bool>::type
-    signbit(_A1) _NOEXCEPT {
-  return false;
-}
-
-#    endif // signbit
-
 // fpclassify
 
 #    ifdef fpclassify
 #      undef fpclassify
+#    endif // fpclassify
 
 template <class _A1>
 inline _LIBCPP_HIDE_FROM_ABI typename std::enable_if<std::is_floating_point<_A1>::value, int>::type
 fpclassify(_A1 __x) _NOEXCEPT {
   return __builtin_fpclassify(FP_NAN, FP_INFINITE, FP_NORMAL, FP_SUBNORMAL, FP_ZERO, __x);
 }
 
 template <class _A1>
 inline _LIBCPP_HIDE_FROM_ABI typename std::enable_if<std::is_integral<_A1>::value, int>::type
 fpclassify(_A1 __x) _NOEXCEPT {
   return __x == 0 ? FP_ZERO : FP_NORMAL;
 }
 
-#    elif defined(_LIBCPP_MSVCRT)
-
-template <typename _A1>
-inline _LIBCPP_HIDE_FROM_ABI typename std::enable_if<std::is_floating_point<_A1>::value, bool>::type
-fpclassify(_A1 __x) _NOEXCEPT {
-  return ::fpclassify(__x);
-}
-
-template <class _A1>
-inline _LIBCPP_HIDE_FROM_ABI typename std::enable_if<std::is_integral<_A1>::value, int>::type
-fpclassify(_A1 __x) _NOEXCEPT {
-  return __x == 0 ? FP_ZERO : FP_NORMAL;
-}
-
-#    endif // fpclassify
-
 // isfinite
 
 #    ifdef isfinite
 #      undef isfinite
 
 template <class _A1>
 inline _LIBCPP_HIDE_FROM_ABI
     typename std::enable_if< std::is_arithmetic<_A1>::value && std::numeric_limits<_A1>::has_infinity, bool>::type