This fixes two compilation errors preventing bootstrap with Ada
on x86_64-pc-cygwin.

2020-05-04  Mikael Pettersson  <mikpelinux@gmail.com>

	PR bootstrap/94918
	* mingw32.h: Prevent windows.h from including emmintrin.h on Cygw64.
	* s-oscons-tmplt.c (Serial_Port_Descriptor): Use System.Win32.HANDLE
	also on Cygwin.