Linux armv7 support about swift-crypto

[Lukasa]

I'd be open to an unofficial docker image, but I don't think hardcoding the symbols themselves is going to work. Another option might be to produce a cross-compilation SDK that supports an armv7 toolchain, which we could then integrate. @MaxDesiatov might be able to help with that.

[MaxDesiatov]

Swift SDKs can be generated from existing toolchains. Thus we need an armv7 toolchain first (for some Linux distribution, Docker images could work too) to run the generator on it.

[orobio]

We don't have a full toolchain for armv7, but we do have a cross-compiling toolchain (Swift 5.10). This was created with something based on the swift-armv7 repo. However, it is for our own Yocto based system. We can have a look at creating a container image with a cross-compiling toolchain for Debian. This should be usable with the vendor-boringssl.sh script. I guess it will not work well with the SDK generator, though.

[Lukasa]

Ideally we'd have either a suitable docker container or a toolchain, I don't much mind. Either way we can add that to the build system once we're there.

[orobio]

A container seems like the best option for now. It fits right into the current set-up and takes away some unknowns regarding SDK creation, especially for us being unfamiliar with Swift on MacOS. I submitted a pull request with a container based solution.