Handle KVM_EXIT_SYSTEM_EVENT on any VCPU

When VCPU #0 exits (e.g. due to KVM_EXIT_SYSTEM_EVENT), it sends
SIGKVMEXIT to all other VCPUs, waits for them to exit, then tears down
any remaining context. The signalling of SIGKVMEXIT is critical to
forcing VCPUs to shut down in response to a system event (e.g. PSCI
SYSTEM_OFF).

VCPUs other that VCPU #0 simply exit in kvm_cpu_thread without forcing
other CPUs to shut down. Thus if a system event is taken on a VCPU other
than VCPU #0, the remaining CPUs are left online. This results in KVM
tool not exiting as expected when a system event is taken on a VCPU
other than VCPU #0 (as may happen if the guest panics).

Fix this by tearing down all CPUs upon a system event, regardless of the
CPU on which the event occurred. While this means the VCPU thread will
signal itself, and VCPU #0 will signal all other VCPU threads a second
time, these are harmless.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Suzuki Poulose <suzuki.poulose@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>