target/ppc: Improve book3s branch trace interrupt for v2.07S

Improve the emulation accuracy of the single step and branch trace
interrupts for v2.07S. Set SRR1[33]=1, and set SIAR to completed
instruction address.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>