SerialPort - initial async implementation on Linux and bugfixes (dotnet/corefx#33027)

SerialPort - initial async implementation on Linux and bugfixes (dotnet/corefx#33027)

* SerialPort - initial async implementation on Linux and bugfixes

* Apply review feedback

* Fix dequeueing on Release builds

* Remove InternalResources.Windows.cs (use Win32Marshal.cs from Common folder)

* Fix build errors on Windows

* fix build errors on uap

* Apply review feedback (+ IOLoop pausing when no activity)

* Fix comment

* apply feedback

* Remove code duplication

Commit migrated from https://github.com/dotnet/corefx/commit/d69f113afb5c43702e7c8477c6dbe2f9e9465355