x/build: add LUCI linux-mips64 builder

[dmitshur]

There currently isn't a LUCI builder that tests the linux/mips64 port (other than the misc-compile builder, which tests only that the port compiles). This is the tracking issue for it.

The next steps that a builder owner will need to follow to make progress here are documented https://go.dev/wiki/DashboardBuilders#luci-builders.

CC @golang/mips.

[dmitshur]

Noting here that if a LUCI builder isn't added to provide test coverage for this port, it risks becoming broken without anyone noticing. There's signal provided by the legacy builders, but it's not viable to rely on them indefinitely because the infrastructure needed to run them is not actively supported past the migration deadline.

CC @golang/mips.