[gn build] Extract compiled_action.gni from tablegen.gni

After years of tablegen being the only host binary we run as part
of the build, we're now in the process of growing at least two more:
- make_confusable_table (https://reviews.llvm.org/rG180bae08a04d)
- pseudo-gen (not yet fully hooked up in the GN build, but I have
  a local branch that hooks it up)

Factor out the few lines we need for running a host binary as part of the
build.

No behavior change.

Differential Revision: https://reviews.llvm.org/D126963