Fix for `is_arm64()` function to support Linux

[eitsupi]

I recommend supporting Linux as well, because when we simply write arm64, it is not limited to macOS.

I tested R on Docker on Raspberry Pi 4.

$ docker run --rm -it r-base R -s -e 'Sys.info()[["machine"]]'
[1] "aarch64"
$ uname -m
aarch64
$ dpkg --print-architecture
arm64