[2020-02][System.Native] Handle ReadDir EINTR (#21029)
* [System.Native] Handle ReadDir EINTR
Handle EINTR for opendir/readdir/closedir.
Because of how thread aborts work, we must loop in managed.
A thread abort will break syscalls and return EINTR, but mono will not raise a
ThreadAbortException until a pinvoke returns.
(Despite the documentation, opendir/readdir/closedir sometimes return EINTR on
macOS Big Sur.)
Related to https://github.com/mono/mono/issues/20799
* bump corefx
* [System] on unix, include Interop.ReadDir.Mono.cs
on mac it's in corlib, on unix and android it's in System, for some reason
* Add newline
Co-authored-by: Alexander Köplinger <alex.koeplinger@outlook.com>