Fix build issue on gcc34

Older versions of gcc may not inline on_extra_stack() and thus cause a
link error when compiling in 32bit segmented mode.  Test for MODE16
explicitly in stack_hop_back() to prevent the problem.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
pciinit: Fix build warning in mch_pci_slot_get_irq()

Some old versions of gcc warn that 'irq might be used uninitialized'.
Replace the switch statement with an if statement to suppress the
warning.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>