[elf] Work around entry point bug in NetBSD kernels
NetBSD kernels are multiboot ELF kernels with an entry point
incorrectly specified as a virtual address rather than a physical
address.
Work around this by looking for the segment that could plausibly
contain the entry point address (interpreted as either a physical or
virtual address), and using that to determine the eventual physical
entry point.
In the event of any ambiguity, precedence is given to interpretation
of the entry point as a physical address.