powerpc: Add support for additional FSCR-managed facilities

Add support to enable, save, and restore the following facilities:

• Target Address Register (bctar) -- seemingly just another register to branch to.
• Event-based branching -- an interrupt-like userspace event handler subsystem.
• Load-monitored facility -- A facility that allows monitoring a range of physical memory, and triggering an event on access. Targeted to garbage collection software features.