Fix style and rename versioned formulae

[samford]

I came across this tap while checking if the ARMmbed/formulae tap had updated to the latest arm-none-eabi version and figured I would help with some improvements here.

This PR does the following:

• Fixes brew style offenses, bringing the formula style up to date.
• Renames arm-gcc-bin@8 to arm-gcc-bin@8-2019-q3-update and arm-gcc-bin@9 to arm-gcc-bin@9-2020-q2-update. The previous formula naming convention would become a problem the next time there's a release in August or September. I figured it would be best to use the full version here, so users can install the exact version they need. [If only the date part was used (e.g., 6-2017) it would only be possible to have one of two releases with the same month/date, like 6-2017-q1-update and 6-2017-q2-update.]
• Adds an arm-gcc-bin@9-2019-q4-major formula for the sake of completeness, so all the stable releases from 2019 onward are available. It's easy to create additional versioned formulae for the remaining releases from the downloads page if anyone has any interest in that.
• Improves the README a bit.

Let me know if this needs any changes.

[ladislas]

Thanks a lot @samford for the PR and the nice cleanup! :)

Are you a user yourself? What's your take on having v10 the default? Should we keep v9 for the moment?

For https://github.com/osx-cross/homebrew-avr, we kept v9 as default to let people experiment with v10 if needed.

For ARM, I've been using v10 since it came out with no significant changes, so I'm happy to keep it as it is.

[samford]

Are you a user yourself? What's your take on having v10 the default? Should we keep v9 for the moment?

I haven't started using this yet, so I don't have any special insight on 9-2020-q2-update vs. 10-2020-q4-major. From a Homebrew perspective, first-party taps (e.g., homebrew/core, homebrew/cask) aim to use the latest version in the formula and sometimes provide versioned alternatives. However, third-party taps can do whatever they feel is most appropriate.

One way to approach this that could make everyone happy is to:

1. Convert arm-gcc-bin to a versioned arm-gcc-bin@10-2020-q4-major formula.
2. Create an Aliases/arm-gcc-bin symlink that points to arm-gcc-bin@10-2020-q4-major.rb.
3. Continue to create a new versioned formula for each release and update the arm-gcc-bin symlink to point to the latest version.

Users who want to always use the latest version can simply install arm-gcc-bin. Alternatively, a user who wants to specifically use 10-2020-q4-major without being updated to a newer version in the future can do so by using brew install arm-gcc-bin@10-2020-q4-major. This would allow users to decide what's most appropriate for their particular circumstance.

I pushed a couple more commits that implement the aforementioned changes, so you can see what I mean and test it out. What do you think about this setup?

[ladislas]

Thanks for the input! I'll review one last time tomorrow morning (it's late in France) and merge your changes. Thanks again for the helpful hand! :)

[leojrfs]

Hey guys, sorry, I've just seen this.

I still find it useful to somehow keep the original versioning format and their respective formulae.

For example, NCS (Nordic's SDK based on zephyr) requires arm-gcc-bin@9-2019-q4-major, as does Contiki. Older Contiki builds for instance, require 5-2015-q4-major.

The great thing about this formulae repo is that all formulae are keg_only, meaning we can have all of those versions installed.

My proposition is then to keep all the formulae in with their original versioning string, but still linking them to the major gcc releases, like so:

• arm-gcc-bin -> arm-gcc-bin@10 -> arm-gcc-bin@10-2020-q4-major
• arm-gcc-bin@10-2020-q2-preview
• arm-gcc-bin@9 -> arm-gcc-bin@9-2019-q4-major
• arm-gcc-bin@9-2019-q4-major
• arm-gcc-bin@8 -> arm-gcc-bin@8-2019-q3-update

and so on...

@ladislas @samford, thoughts?