add support for darwin/arm64 (Apple Silicon)

[joelanford]

This PR adds support for Apple Silicon by adding darwin/arm64 to the build matrix.

The install scripts are a bit more complicated now because uname -m on Apple Silicon returns arm64, so the sed -e 's/arm.*$/arm/' substitution resulted in darwin_arm, rather than darwin_arm64.

I fixed that as best I could with a case statement, but perhaps there's a simpler approach. I looked into a pure sed solution, but it doesn't seem like sed supports the necessary lookahead features that would enable search/replace for arm + anything other than 64.

Fixes #686

[ahmetb]

this part is a little suspicious: why did you need case "${OS}" in "darwin") ?

it seems that uname -m already returns arm64 in Apple Silicon chip, so it would work as is?

if you did this because of -e 's/arm.*$/arm/', that's understandable as it could return values like armv7l, armv8 in other platforms such as Linux. in that case, we can rewrite the regexp pattern in a way that does not replace amd64 (coming from M1).

[joelanford]

Yeah, this is the part that I mentioned in the PR description.

The install scripts are a bit more complicated now because uname -m on Apple Silicon returns arm64, so the sed -e 's/arm.*$/arm/' substitution resulted in darwin_arm, rather than darwin_arm64.

I fixed that as best I could with a case statement, but perhaps there's a simpler approach. I looked into a pure sed solution, but it doesn't seem like sed supports the necessary lookahead features that would enable search/replace for arm + anything other than 64.

Any better ideas?

[joelanford]

Pushed a new update with a better sed-only replacement that seems to work. WDYT?

[ahmetb]

Looks good. Thanks.
/lgtm
/approve

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