Fix Bootstrapping on Apple Silicon

[thomashope]

What does this PR do?

When following the instructions Building Premake on an Apple Silicon Mac the build fails due to the default architecture being x86.

This PR updates the documentation to inform Apple Silicon users to set PLATFORM when building, and adds a check to Bootstrap.sh which should detect Apple Silicon hardware and set PLATFORM automatically if not explicitly set by the user.

How does this PR change Premake's behavior?

This should no change Pemake's behaviour.

Anything else we should know?

In BUILD.txt I change some wording from PLATFORM to HOST_PLATFORM to a) distinguish it from the PLATFORM variable (which might be better named ARCH), and b) to match the help text at the top of Bootstrap.mak.

Split into two commits to separate build script changes and documentation changes.

Did you check all the boxes?

• Focus on a single fix or feature; remove any unrelated formatting or code changes
• Add unit tests showing fix or feature works; all tests pass
• Mention any related issues (put closes #XXXX in comment to auto-close issue when PR is merged)
• Follow our coding conventions
• Minimize the number of commits
• Align documentation to your changes

You can now support Premake on our OpenCollective. Your contributions help us spend more time responding to requests like these!

[thomashope]

Looks like the macOS CI actions are listed as failed, but they were actually canceled?

Anything I can do about that or does someone need to give them a kick?

[Jarod42]

Error stated

The job was not acquired by Runner of type hosted even after multiple attempts

nickclark2016 just relaunched them.

[nickclark2016]

Error stated

The job was not acquired by Runner of type hosted even after multiple attempts

nickclark2016 just relaunched them.

GH Actions for Mac Runners are currently degraded