[libcxx] Use the allocating form of getcwd() on Glibc and Apple platforms

[mstorsjo]

Summary

This avoids having to query pathconf for a max size for preallocating a buffer for the return value.

This is an extension to the POSIX getcwd() spec.

[ldionne]

LGTM pending CI.

libcxx/src/filesystem/operations.cpp

void __create_symlink(path const& from, path const& to, error_code* ec) {
   ErrorHandler<void> err("create_symlink", ec, &from, &to);
   if (detail::symlink_file(from.c_str(), to.c_str()) == -1)
     return err.report(capture_errno());
 }
 
 path __current_path(error_code* ec) {
   ErrorHandler<path> err("current_path", ec);
 
-#if defined(_LIBCPP_WIN32API)
+#if defined(_LIBCPP_WIN32API) || defined(__GLIBC__) || defined(__APPLE__)
   // Common extension outside of POSIX getcwd() spec, without needing to
   // preallocate a buffer. Also supported by a number of other POSIX libcs.
   int size = 0;
   path::value_type* ptr = nullptr;
   typedef decltype(&::free) Deleter;
   Deleter deleter = &::free;
 #else
   auto size = ::pathconf(".", _PC_PATH_MAX);