target/ppc: PMU implement PERFM interrupts

target/ppc: PMU implement PERFM interrupts

The PMU raises a performance monitor exception (causing an interrupt
when MSR[EE]=1) when MMCR0[PMAO] is set, and lowers it when clear.

Wire this up and implement the interrupt delivery for books. Linux perf
record can now collect PMI-driven samples.

fire_PMC_interrupt is renamed to perfm_alert, which matches a bit closer
to the new terminology used in the ISA and distinguishes the alert
condition (e.g., counter overflow) from the PERFM (or EBB) interrupts.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20230530134313.387252-2-npiggin@gmail.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>