Preliminary Fedora cygwin-aarch64 cross toolchain

[Jon Turney]

To facilitate adding aarch64 builds to our CI, and so I can experiment 
with how to add some initial aarch64 packages to our packaging pipeline, 
I've created a new copr (Fedora package repository) at:

https://copr.fedorainfracloud.org/coprs/g/cygwin/cygwin/

(The intent is that this can eventually replace the long-standing, 
semi-official copr at 
https://copr.fedorainfracloud.org/coprs/yselkowitz/cygwin/)


I've imported the history of the packages there into a set of 
repositories you can find at:

https://cygwin.com/cgit/cygwin-rpm-copr/

(If you have a sourceware account with cygwin group membership, you can 
and should push updates there. There is a post-update hook in place 
which automatically triggers a build to update the package on copr)

(Additionally, for the cygwin and cygwin-default-manifest packages, 
there are special instructions in .copr/Makefile to teach it how to 
fetch the upstream source from git, since tar archives are not available)


I've finished off and cleaned up some work I did over this summer to add 
to those packages support for building a cross-toolchain on Fedora which 
targets cygwin-aarch64.

(Thanks to eukarpov for curating the binutils and gcc patchsets which I 
used. These patchsets still need updating to the current state of 
development).