Moved hostArch() earlier in validateListFromOptions()

[mathuin]

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• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

This PR is in response to atom/atom#15431. When I tried to build Atom on my Chromebook running Arch Linux ARM, the automatic trigger detecting armv7l was not running because a value already existed for the arch. This change moves the hostArch() function before the list assignment to catch this case.

I did not add documentation for this change because it is sufficiently simple that I think additional comments would not be helpful. Similarly, I did not add tests. I confess I don't know how to run the test suite, but the code did pass one important test -- when I ran it, the correct architecture was detected and
a package was built!

[welcome]

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[malept]

The big problem with this solution is that the tests fail. Frankly, I'm not sure it's the correct solution.

[mathuin]

That's fine -- the intent was to share with you the code that ran on my system in the hopes that you would be able to make a proper patch. Like I mentioned in the other issue, I don't know the language and certainly don't understand the codebase. It's okay if you want to close the PR -- I just wanted you to see the code that worked here.

[malept]

I'm going to close this due to the rationale I laid out in atom/atom#15431 (comment) - in the near future, I'll add explicit unit tests for hostArch() and validateListFromOptions() regarding armv7l detection.