Windows support

[torkelrogstad]

Adds support for building Windows binaries. This happens through cross-compiling from Linux. Some notes on this:

1. I don't have a totally precise understanding of exactly what's needed on the builder machine. There are some notes scattered around the Zcash repo. I ended up using a Docker image provided by the Zcash team: electriccoinco/zcashd-build-ubuntu-jammy. I was not able to find the source for this Docker image, and therefore do not know precisely what it contains.
2. The same build instructions we use for Linux and macOS work, with the sole exception of having to also link against libssp. This is needed in order to find a hardened version of memcpy.
3. I have not been able to actually run this binary successfully. It runs for a few seconds, and then exits with a segmentation fault. Will have to dig into the underlying issues of this.

# from the root of this repo
docker run -ti -v $PWD:/zside --workdir /zside electriccoinco/zcashd-build-ubuntu-jammy bash

HOST=x86_64-w64-mingw32 make -C depends V=1 -j8
export CONFIG_SITE=$PWD/depends/x86_64-w64-mingw32/share/config.site
./autogen.sh
./configure --disable-tests --disable-bench --disable-hardening --enable-online-rust
make -C src cargo-build-lib
BRIDGE_LOCATION=$(dirname $(./contrib/devtools/find-libcxxbridge.sh))
export LDFLAGS="-L$BRIDGE_LOCATION -lcxxbridge1" 
./configure --disable-tests --disable-bench --disable-hardening --enable-online-rust
make -j8

# final result is in ./src/zsided.exe

[Ash-L2L]

LGTM