Add Snap build for `armhf`/`armv7` architecture

[danielpgross]

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Attempting to build the snap on Snapcraft Cloud for armhf produces the following error:

zig build --global-cache-dir /build/friendly-neighbor/parts/friendly-neighbor/build/zig-cache/
thread 9738 panic: integer overflow
/snap/zig/8738/lib/std/process.zig:1222:22: 0xb6a43 in totalSystemMemoryLinux (build)
    return kilobytes * 1024;
                     ^
/snap/zig/8738/lib/std/process.zig:1164:42: 0xa578b in totalSystemMemory (build)
            return totalSystemMemoryLinux() catch return error.UnknownTotalSystemMemory;
                                         ^
/snap/zig/8738/lib/build_runner.zig:316:48: 0xa1dcb in main (build)
        run.max_rss = process.totalSystemMemory() catch std.math.maxInt(usize);
                                               ^
/snap/zig/8738/lib/std/start.zig:370:37: 0x904e3 in posixCallMainAndExit (build)
            var i: usize = 0;
                                    ^
???:?:?: 0x0 in ??? (???)
error: the following build command crashed:
/build/friendly-neighbor/parts/friendly-neighbor/build/zig-cache/o/4fb5c8308bb66929791e1c0a7a99fb3b/build /snap/zig/8738/zig /build/friendly-neighbor/parts/friendly-neighbor/build /build/friendly-neighbor/parts/friendly-neighbor/build/zig-cache /build/friendly-neighbor/parts/friendly-neighbor/build/zig-cache/
'override-build' in part 'friendly-neighbor' failed with code 1.

Seems to be caused by https://github.com/ziglang/zig/issues/17596.

We should ideally support armhf, since many low-power devices use it, including Raspberry Pi version 2 and earlier.

[CommanderTso]

Just ran into this trying to install on my Pi 3; I'd love to be able to use this on there. Thanks.

[samwinslow]

I don't mean to take focus away from this project and specific issue, but I ran into the same difficulties trying to get the toolchain set up on armv6l (Pi Zero W) and decided the most expedient thing to do was rewrite the basic functionality in C.

See @samwinslow/arpwake. It is currently IPv4 only; it also seems to work just fine on the Zero W's wireless interface.