[libc++] Fix build with gcc 4.8

gcc 4.8.4 (but not 5.4.0 or 7.3.0) has trouble initializing errc with {}, giving
the error in [1]. This CL switches to explicitly using errc(0), which gcc 4.8
accepts.

[1] https://bugs.chromium.org/p/chromium/issues/detail?id=973723

Differential Revision: https://reviews.llvm.org/D63296

llvm-svn: 363333

libcxx/include/charconv

@@ -314,7 +314,7 @@ __to_chars_itoa(char* __first, char* __last, _Tp __value, false_type)
 
 #if !defined(_LIBCPP_COMPILER_MSVC)
     if (__tx::digits <= __diff || __tx::__width(__value) <= __diff)
-        return {__tx::__convert(__value, __first), {}};
+        return {__tx::__convert(__value, __first), errc(0)};
     else
         return {__last, errc::value_too_large};
 #else