Initiate de-orbit burn for USE_PCI_BIOS_FOR_READ_WRITE. This has been
#if'ed out for a while. Complete the deed and tidy up some other bits.

We need to be able to call this stuff from outer edges of interrupt
handlers for devices that have the ISR bits in pci config space. Making
the bios code mpsafe was just too hairy. We had also stubbed it out some
time ago due to there simply being too much brokenness in too many systems.
This adds a leaf lock so that it is safe to use pci_read_config() and
pci_write_config() from interrupt handlers. We still will use pcibios
to do interrupt routing if there is no acpi.. [yes, I tested this]

Briefly glanced at by: imp