profiles: new arch and profile for x64-winnt

profiles: new arch and profile for x64-winnt

New profile prefix/windows/winnt/x64, for new ARCH=x64-winnt.

This profile is designed for use as Stacked Prefix, set up using
prefix-stack-setup (app-portage/prefix-toolkit) from with a Cygwin
Prefix (Guest only), having sys-devel/parity installed (wraps MSVC).

CHOST is left unset on purpose.  It is set by prefix-stack-setup
using some --chost=x86_64-msvc16-winnt argument, selecting the
Visual Studio version and runtime variant (static|debug) to use.

Adding same comments to prefix/windows/winnt/x86 profile as well.

Signed-off-by: Michael Haubenwallner <haubi@gentoo.org>