Add ARM64 Build

[yalh76]

Include an ARM64 build

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• I have signed the CLA

[thebaer]

Thanks for submitting this! Out of curiosity, is there anything in particular you need the ARM64 build for?

[yalh76]

Yes, to make it available for the WriteFreely package for YunoHost.
A lot of YunoHost users are on RaspberryPi.

Since version 0.8.1, I provide a writefreely build for ARM64.
And I've seen you added a build for arm7 at version 0.10.0.

So that time, when I was working on v0.11.1 for YunoHost and building the ARM64 version, that took me a lot of time (as I was building it on a fresh debian ARM64), I was writing a script to automate the build for ARM64, also trying to do cross-compilation to gain time from another AMD64 VM with no luck. I dived in the WriteFreely makefile, and though It would be easier for me to have the ARM64 version directly available from you ^^

[thebaer]

Definitely makes sense! Happy to include that as part of our build process -- did it for our v0.11.2 release.

Thanks again for contributing this! Merging now.