Use MMIO mechanism to access PCI [extended] config space on x86_64

[kevinaboos]

#1019 added support for accessing the PCI config space using MMIO, which is much better than the legacy method of accessing it using Port I/O. Also, it can support the extended PCI config space which is needed for PCIe and newer versions of that, so we should definitely prefer using MMIO to access the PCI config space in all cases except when it is not supported (only really an issue on ancient legacy hardware).