Add `arch` prompt for displaying CPU architecture

[brent-moffit]

Adds the arch prompt to p10k.zsh, ARCH_ICON to icons.zsh, and commented entries in the default configs.

Displaying the current CPU architecture is useful on systems where shells may be opened under emulation. For example, ZSH is commonly run using Rosetta on M1-based macs to emulate i386/x86_64 systems.

Users can optionally set POWERLEVEL9K_ARCH_DEFAULT to hide the prompt in shells running under the default system architecture.

[ianks]

This is awesome.

[kdekorte]

Can we get this merged it, it would be great for working on my Mac where I have terminals with different arch's running

[romkatv]

Merged. Note that I renamed arch to cpu_arch, removed POWERLEVEL9K_ARCH_DEFAULT and added state. If you've been using the code from the PR, you'll need to update .p10k.zsh.