@prelude//os_lookup/targets:os_lookup incomplete

In [//decls:common.bzl](https://github.com/facebook/buck2-prelude/blob/main/decls/common.bzl#L200), _target_os_type_arg returns [//os_lookup/targets:os_lookup](https://github.com/facebook/buck2-prelude/blob/main/os_lookup/targets/BUCK#L8), which puts "linux" as the platform when not specifically Windows or macOS. The rule also accepts "freebsd" and "unknown". It looks like the cpu bit is incomplete as well, accepting arm, armv7, arm64, x86, x86_64, and mips but only having branches for arm64 and x86_64.

None technically isn't a value for TargetCpuType but is used as a branch here. Could the same be used for config//os:none?

Start with _target_os_type_arg in decls/common.bzl and the mappings in os_lookup/targets/BUCK. Compare the accepted OS and CPU values with the existing branches, including the config//os:none question, and verify that each intended value resolves to the expected target.

Written by the indexing model from the issue text.