Auto merge of #19701 - ntenczar:update-readme-clang, r=jdm

Update Arch Linux setup to include clang

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`clang` is needed to compile servo, but isn't listed as one of the dependencies needed for arch and isn't part of the `base-devel` group.

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- [X] `./mach test-tidy` does not report any errors
- [X]  no applicable github issue

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- [X] These changes do not require tests because it's a README change

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