powerpc backend is missing bctar and related branch mnemonics

`bctar` was added in Power ISA 2.07 but isn't supported by the powerpc backend. We'll also want to add related mnemonics like `blttarl-` or `bdztar+`.

The requested entry point is the powerpc backend; start by locating its instruction definitions and existing tests for branch mnemonics. Confirm how bctar and related forms such as blttarl- and bdztar+ are represented, then verify the backend accepts them and the relevant tests pass.

Written by the indexing model from the issue text.