Fix avr-gcc@5 build

[fauxpark]

Attempting to fix this formula in order to clear #324

[fauxpark]

Despite the green, the build is failing. Since Homebrew have now disabled isl@0.18 I suspect it may not be possible to build GCC 5 and this formula might need to be removed.

[ladislas]

we can keep it unbottled. people really needing this one can still build from source.
let's focus our effort on the most used versions first :)

[ladislas]

as you can see here, there are not a lot of users for this version.

https://qii404.me/github-release-statistics/

or here https://tooomm.github.io/github-release-stats/?username=osx-cross&repository=homebrew-avr

[ladislas]

@fauxpark I've removed gcc@5 from the repo with #345

since the last release in Feb 2021 there has only been 221 downloads

the problem with building comes from the fact that isl@0.18 has been deprecated. I'm not sure it's worth the effort to also maintain isl@0.18 locally.

If you're okay, we can close this PR

[fauxpark]

Sounds good to me.