x86, ioapic: Improve handling of i8259A irq init

Since we already track the number of legacy vectors by nr_legacy_irqs, we
can avoid use static vector allocations -- we can use dynamic one.

Signed-off-by: Alek Du <alek.du@intel.com>
LKML-Reference: <43F901BD926A4E43B106BF17856F07559FB80D01@orsmsx508.amr.corp.intel.com>
Signed-off-by: Jacob Pan <jacob.jun.pan@intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>