Add ARM kCpuArchitecture string for Linux platforms

[davidalo]

Description

ARM architecture is working as expected for me using Auth and Database services, the unique problem I had was the kCpuArchitecture string declaration

[Add kCpuArchitecture]: # Add ARM kCpuArchitecture string for Linux platforms

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Testing

Manually tested using examples.

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Type of Change

Place an x the applicable box:

• [ ] Bug fix. Add the issue # below if applicable.
• [x] New feature. A non-breaking change which adds functionality.
• [ ] Other, such as a build process or documentation change.

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Notes

• Bug fixes and feature changes require an update to the Release Notes section of release_build_files/readme.md.
• Read the contribution guidelines CONTRIBUTING.md.
• Changes to the public API require an internal API review. If you'd like to help us make Firebase APIs better, please propose your change in a feature request so that we can discuss it together.

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[davidalo]

I signed CLA, it should be ready.