Use uname -m instead of arch comamnd

[conao3]

arch command is not installed at default.
see coreutils document:

arch is not installed by default, so portable scripts should
not rely on its existence.

https://www.gnu.org/software/coreutils/manual/html_node/arch-invocation.html

ref: google/fscrypt#92 (comment)

closes: #5629

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Note: Documentation fixes for https://docs.haskellstack.org/en/stable/ should target the "stable" branch, not master.

Please include the following checklist in your PR:

• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary.

Please also shortly describe how you tested your change. Bonus points for added tests!

[hasufell]

Also relevant: https://superuser.com/questions/835514/why-do-uname-p-and-uname-m-and-arch-output-different-architectures