x/build/env/fuchsia: add new Fuchsia builder

[bradfitz]

@dhobsd is planning on adding a Fuchsia port.

Fuchsia is not yet self-hosting, so the builder plan is to use a linux/amd64 host to do the build with nested virtualization enabled and run qemu+kvm to run the tests.

/cc @dhobsd @dmitshur

[gopherbot]

Change https://golang.org/cl/162959 mentions this issue: dashboard, buildlet: add a disabled builder with nested virt, for testing

[gopherbot]

Change https://golang.org/cl/163005 mentions this issue: env: add Dockerfile for generating a Fuchsia buildlet

[gopherbot]

Change https://golang.org/cl/163057 mentions this issue: buildlet: change image name for COS-with-vmx buildlet

[gopherbot]

Change https://golang.org/cl/163301 mentions this issue: env/linux-x86-vmx: add new Debian host that's like Container-Optimized OS + vmx