Create kernel compile action

[cbracken]

Along with the checked in sources in the repo, Bazel emits generated code to either bazel-bin or bazel-genfiles. Since checked-in code frequently imports generated code, toolchain support for multiple parallel source roots is required.

Since no such support previously existed, layout_action was added to generate a symlink tree. The kernel compiler does however support this via the --multi-root option.

We should extract out a kernel compile action then incorporate it to the existing rules.