Add Arch and Manjaro to supported OSes

What does this Merge Request do?

Disclaimer: I'm not an engineer, so if what I'm doing here is awful practices, please tell me.

I'm currently on Manjaro, which is based on Arch Linux, and gdk update fails for me, because it runs ensure_supported_platform in bootstrap-common.sh.

If I remove this check from asdf-update and run bash support/asdf-update, asdf goes on to update the tool versions:

Command output

$ bash support/asdf-update

--------------------------------------------------------------------------------
Ensuring platform software installed
--------------------------------------------------------------------------------

--------------------------------------------------------------------------------
Updating asdf plugins
--------------------------------------------------------------------------------
Updating ruby...
Already on 'master'
Your branch is up to date with 'origin/master'.
Updating golang...
Already on 'master'
Your branch is up to date with 'origin/master'.
Updating nodejs...
Already on 'master'
Your branch is up to date with 'origin/master'.
Updating yarn...
Already on 'master'
Updating minio...
Already on 'master'
Your branch is up to date with 'origin/master'.
Updating postgres...
Already on 'master'
Your branch is up to date with 'origin/master'.
Updating redis...
Already on 'master'
Your branch is up to date with 'origin/master'.

--------------------------------------------------------------------------------
Updating asdf tools
--------------------------------------------------------------------------------
golang 1.15.5 is already installed
minio 2020-10-03T02-19-42Z is already installed
nodejs 12.18.4 is already installed
nodejs 14.15.4 is already installed
postgres 11.9 is already installed
postgres 12.4 is already installed
redis 6.0.8 is already installed
ruby 2.7.2 is already installed
ruby 3.0.0 is already installed
yarn 1.22.5 is already installed

This adds Arch and Manjaro and their package manager (pacman) to the update script.

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.