Fixes regression introduced by bitcoin#16141

-BEGIN VERIFY SCRIPT-
sed -i 's#\$REFERENCE_DATE\\\\\\ \$REFERENCE_TIME#\$REFERENCE_DATETIME#g' contrib/gitian-descriptors/*
-END VERIFY SCRIPT-

This workaround was added as part of the switch to gitian building using Ubuntu 14.04 (dashpay#6900).
However, it should no longer be required, as we have switched to Bionic (bitcoin#13171), and that
has a far newer version of binutils.

binutils patch: https://sourceware.org/bugzilla/show_bug.cgi?id=16192

Debug splitting was first introduced in 7e7eb27, then gitian-linux.yml
changed to using split-debug.sh in 9d25362. Here we change
gitian-win.yml to use split-debug.sh as well.

Linux:
The README was originally added in 8550f1f, but included the README
under the docs directory, which has a bunch of internal links that won't
make sense in a release tarball. In this patch, we include the root
level README instead, which makes more sense.

Windows:
.md files are inconvenient to open on windows and the line endings
differ, so we use README_windows.txt instead.

We no longer build/ship 32 bit windows executables.

configure.ac defines the `PACKAGE_URL` variable during init, so use that
in the windows packaging nsi script instead of a static string.