x/build: add darwin-arm64 longtest and race builders

[FiloSottile]

Per #69593 (comment), there should now be the capacity for longtest and race darwin-arm64 builders.

They would be especially valuable because the darwin/arm64 machines are our most capable ARM64 builders, and some cryptographic assembly is only tested on them (#69593).

/cc @golang/release

[dmitshur]

CC @JunyangShao, @prattmic.

[gopherbot]

Change https://go.dev/cl/667935 mentions this issue: main.star: add the darwin-arm64-longtest and race builder type

[gopherbot]

Closed by merging CL 667935 (commit golang/build@f0d5297) to luci-config.