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

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.

Differential Revision: https://reviews.llvm.org/D97460

Author: mstorsjo

libcxx/src/filesystem/operations.cpp

@@ -1095,7 +1095,7 @@ void __create_symlink(path const& from, path const& to, error_code* ec) {
 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;