x/build: misc-compile coverage for ios/arm64 port (iOS) in LUCI

This is like #25963, but for LUCI.

The current failure mode:

```
# bufio.test
loadinternal: cannot find runtime/cgo
/.../goroot/pkg/tool/linux_amd64/link: running clang failed: exec: "clang": executable file not found in $PATH
```

See https://logs.chromium.org/logs/golang/buildbucket/cr-buildbucket/8773728633417673633/+/u/step/10/log/2.

CC @golang/ios.