Upstream a bug fix for platform detection on MacOS

[davidanthoff]

The Julia programming language has a tool Juliaup which was not only heavily inspired by rustup, but I also copied the rustup-init.sh script as a starting point. First of all: thanks so much for an awesome project, the inspiration and the code, it made building Juliaup sooo much easier!

We recently fixed a pretty bizzar corner case bug with the shell script re platform detection, and I just thought I'd let you know in case you want to copy that over into your rustup-init.sh :) The PR is JuliaLang/juliaup#701.

[StefanKarpinski]

Might be nice to factor some of the gnarly platform-detection functionality into a separate file that can be shared between the languages so that fixes are easier to share, but maybe would add too much overhead.

[rbtcollins]

This would be great to turn into a PR - thank you!

[rbtcollins]

@davidanthoff thank you for this - you might want to see our diff - we found additional corner cases in the approach in your patch.