x86, acpi/irq: Generalize mp_config_acpi_legacy_irqs

Remove the assumption that there is not an override for isa irq 0.
Instead lookup the gsi and from that lookup the ioapic and pin of each
isa irq indivdually.

In general this should not have any behavioural affect but in
perverse cases this gets all of the details correct, instead of
doing something weird.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
LKML-Reference: <1269936436-7039-5-git-send-email-ebiederm@xmission.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>