Add support for arm64 architecture requirement

[EricFromCanada]

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Allow formulae to specify depends_on arch: :arm64 for ARM-specific packages.

[fxcoudert]

• Do we support 32-bit ARM?
• If :intel means either :i386 or :x86_64, should :arm mean either :arm32 or :arm64?

[EricFromCanada]

* Do we support 32-bit ARM?

Not for macOS, obviously, but it seems we do on Linux, in theory anyway.

* If `:intel` means either `:i386` or `:x86_64`, should `:arm` mean either `:arm32` or `:arm64`?

Yes. My guess is that there won't be many ARM-only packages that compile only on 64-bit ARM and not 32-bit; only those would be marked :arm64, any that run on either would be marked :arm. Willing to take a wait-and-see approach here.

[MikeMcQuaid]

Thanks again @EricFromCanada!