Add support for Amazon A1 instances

This is a tracking issue to enable LinuxKit arm64 support for the recently announced arm64 instances.

For now there seems to be two work items

• Update aws-sdk-go to at least v1.15.83 which seems to have added arm64 support
• Adjust linuxkit aws subcommands to support A1
• Enable the ENA driver for arm64 (A1 instances use ENA for networking)

The last point is a bit tedious as 8c590f977638 ("net: ena: Fix Kconfig dependency on X86") is not in any stable release and there were around 20 patches to the ENA driver since 4.19.12.

We can either wait for 4.20.x to be out (should be soon) or cherry pick the patches into 4.19.x.

@justincormack any preference?

4.20 will be out by the end of the year (Linus likes Christmas day releases), so seems ok to wait for that (I guess it may get backported later anyway).

Looks like 4.20 was released today. Will give that a shot soon

What's left to do on this? I'd really like to use the m6g instances which I'm guessing are a similar problem? I'm happy to contribute to the solution and help if someone can point me in the right direction.

As far as I know no work was done on this.

just FYI, this is an issue for not just arm64, but also for x86_64 systems as well. t3, t3a, and potentially others now come with enhanced networking, and apparently the AMI needs to be enabled for ENA

UPDATE: hm, apparently linux push aws has a -ena option! I've submitted #3829 to fix this