[libcxx] Add missing _LIBCPP_FUNC_VIS on a couple win32 locale functions

[mstorsjo]

Summary

These functions are called directly from the public installed headers, and thus need to be exported in DLL builds, just like some other functions in the same header (e.g. snprintf_l).

This fixes e.g. test/std/localization/locale.categories/category.numeric/locale.num.get/facet.num.get.members/get_float.pass.cpp in mingw configurations.

libcxx/include/support/win32/locale_win32.h

 #define mbtowc_l _mbtowc_l
 #define strtoll_l _strtoi64_l
 #define strtoull_l _strtoui64_l
 #define strtod_l _strtod_l
 #if defined(_LIBCPP_MSVCRT)
 #define strtof_l _strtof_l
 #define strtold_l _strtold_l
 #else
-float strtof_l(const char*, char**, locale_t);
-long double strtold_l(const char*, char**, locale_t);
+_LIBCPP_FUNC_VIS float strtof_l(const char*, char**, locale_t);
+_LIBCPP_FUNC_VIS long double strtold_l(const char*, char**, locale_t);
 #endif
 inline _LIBCPP_INLINE_VISIBILITY
 int
 islower_l(int c, _locale_t loc)
 {
  return _islower_l((int)c, loc);
 }