Step to move shared irq handling out of core in the long run. So, use
irq_proxy implementation from base in os and implement shared irq handling
in platform driver of x86 (pci_drv).
Fixes#1471
This patch changes the top-level directory layout as a preparatory
step for improving the tools for managing 3rd-party source codes.
The rationale is described in the issue referenced below.
Issue #1082