feat: add emulator constraint for QEMU

Previously, we were registering the QEMU toolchain against any AMD64
Linux, which would select that toolchain if running one of the workflows
(e.g. `labgrid_genrule`) without specifying a platform. We now require
consumers to opt-in to using QEMU by using the constraint
`@rules_labgrid//constraint/emulator:qemu`.

The new constraint lives under a new top-level directory, as it's not
specific to `rules_labgrid` and could be extracted into a separate
module in the future.