add support for riscv64

[aptalca]

This PR adds the jenkins-vars.yml argument build_riscv64 (default false)

When set to true, it does the following:

• Builds Dockerfile.riscv64 on native riscv64 builder and pushes to riscv64 tag
• Adds the green checkmark for riscv64 under Available Architectures in the readme (if set to false, it does not add a red X, so existing readmes without riscv64 are not modified) locally tested:
  

Requires:

• MULTIARCH=true in jenkins-vars.yml (not able to build only amd64 and riscv64)

If build_riscv64 is not defined or set to false, it does not modify existing readmes or Jenkinsfiles so this PR shold not have any impact on existing repo templates (locally tested)

Test results:
baseimage-alpine:edge dev build with locally generated Jenkinsfile: https://ci.linuxserver.io/blue/organizations/jenkins/Docker-Pipeline-Builders%2Fdocker-baseimage-alpine/detail/edge-riscv64/1/pipeline/
CI test results: https://ci-tests.linuxserver.io/lsiodev/alpine/edge-67abfac6-pkg-67abfac6-dev-df89f330680499dc1d4c0a487ef1b8acb58b4bf4/index.html

docker-ci container is already updated to support riscv64 as a valid platform.

One decision that needs to be made by the team is w/ regards to qemu builds. Currently our ci allows for building arm64 on X86_64 hw via qemu by setting use_qemu to true. This PR uses the same functionality for riscv64 as well. However, due to limited hw availability for riscv64 (and the surprisingly good qemu build performance), we may wish to have native arm64 but qemu riscv64 for some repos. If that's desired, I can add a new argument, perhaps use_qemu_riscv64? Let me know
I went ahead and added or statements to both arm64 and riscv64 build blocks with new optional args use_qemu_arm64 and use_qemu_riscv64 so that we can have them use qemu for one or both.

• use_qemu: true both arm64 and riscv64 use qemu
• use_qemu_arm64: true only arm64 uses qemu
• use_qemu_riscv64:true only riscv64 uses qemu

[LinuxServer-CI]

I am a bot, here are the test results for this PR:
https://ci-tests.linuxserver.io/lspipepr/jenkins-builder/957bc8d6-pkg-957bc8d6-dev-00d339ed1d89d7ba8f14311f96d2534c0a49ee1c-pr-335/index.html
https://ci-tests.linuxserver.io/lspipepr/jenkins-builder/957bc8d6-pkg-957bc8d6-dev-00d339ed1d89d7ba8f14311f96d2534c0a49ee1c-pr-335/shellcheck-result.xml

Tag	Passed
amd64-957bc8d6-pkg-957bc8d6-dev-00d339ed1d89d7ba8f14311f96d2534c0a49ee1c-pr-335	✅
arm64v8-957bc8d6-pkg-957bc8d6-dev-00d339ed1d89d7ba8f14311f96d2534c0a49ee1c-pr-335	✅

[LinuxServer-CI]

I am a bot, here are the test results for this PR:
https://ci-tests.linuxserver.io/lspipepr/jenkins-builder/957bc8d6-pkg-957bc8d6-dev-6380cd485fc3c0724f2c4461555423fb4e908707-pr-335/index.html
https://ci-tests.linuxserver.io/lspipepr/jenkins-builder/957bc8d6-pkg-957bc8d6-dev-6380cd485fc3c0724f2c4461555423fb4e908707-pr-335/shellcheck-result.xml

Tag	Passed
amd64-957bc8d6-pkg-957bc8d6-dev-6380cd485fc3c0724f2c4461555423fb4e908707-pr-335	✅
arm64v8-957bc8d6-pkg-957bc8d6-dev-6380cd485fc3c0724f2c4461555423fb4e908707-pr-335	✅