x/build: add LUCI linux-arm builder

[dmitshur]

Planned hostname prefix is linux-arm-aws, with the individual builders having a --nn suffix.

Edit: This builder will also serve as a security trybot. A separate set of instances will be used for that, using linux-arm-aws-security as their hostname prefix.

[gopherbot]

Change https://go.dev/cl/558198 mentions this issue: main.star: add linux-arm builder

[gopherbot]

Change https://go.dev/cl/558516 mentions this issue: WIP: env/linux-arm/aws: update to connect to LUCI

[gopherbot]

Change https://go.dev/cl/558517 mentions this issue: main.star: fix cipd_platform value for GOHOSTARCH=arm

[gopherbot]

Change https://go.dev/cl/559158 mentions this issue: main.star: use linux-arm64 cipd_platform dimension for linux-arm host type