PPC64/TCG: Implement 'rfebb' instruction

PPC64/TCG: Implement 'rfebb' instruction

An Event-Based Branch (EBB) allows applications to change the NIA when a
event-based exception occurs. Event-based exceptions are enabled by
setting the Branch Event Status and Control Register (BESCR). If the
event-based exception is enabled when the exception occurs, an EBB
happens.

The following operations happens during an EBB:

- Global Enable (GE) bit of BESCR is set to 0;
- bits 0-61 of the Event-Based Branch Return Register (EBBRR) are set
to the the effective address of the NIA that would have executed if the EBB
didn't happen;
- Instruction fetch and execution will continue in the effective address
contained in the Event-Based Branch Handler Register (EBBHR).

The EBB Handler will process the event and then execute the Return From
Event-Based Branch (rfebb) instruction. rfebb sets BESCR_GE and then
redirects execution to the address pointed in EBBRR. This process is
described in the PowerISA v3.1, Book II, Chapter 6 [1].

This patch implements the rfebb instruction. Descriptions of all
relevant BESCR bits are also added - this patch is only using BESCR_GE,
but the next patches will use the remaining bits.

[1] https://wiki.raptorcs.com/w/images/f/f5/PowerISA_public.v3.1.pdf

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Matheus Ferst <matheus.ferst@eldorado.org.br>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20211201151734.654994-9-danielhb413@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>