[Alpine Linux] `s390x` platform support

## Description
Dotnet6 on Fedora supports `s390x`. Dotnet6 on Alpine should do so, too.

Parallel thread on [Alpine Gitlab](https://gitlab.alpinelinux.org/alpine/aports/-/issues/13685)

## Major roadblocks
As there are no dotnet6 prebuilt bootstraps release for `s390x`, we have to build our own by crosscompiling from `x86_64` to `s390x`. Crosscompilation scripts was kindly provided by the Fedora team, and added to a git repo on Apine's Gitlab instance for further work.

## Hypothetical steps
1) clone aports, and cd `aports/community/dotnet6-stage0`
2) `abuild deps` to install build dependencies of dotnet
3) `apk add quilt mono dotnet6-sdk`
4) `git clone https://gitlab.alpinelinux.org/ayakael/dotnet6-s390x` (original version [here](https://github.com/dotnet/source-build/files/8463748/dotnet-s390x.tar.gz))
5) `cd dotnet6-s390x` && `./dotnet-s390x-prepare`
6) `./dotnet-s390x-build`

cc. @nekopsykose @uweigand @omajid