Make __startkernel line up with KERNBASE, so that the math to compute the
applied relocation offset in link_elf.c works as intended. We may want to
revisit how that works in future, for example by having elf_reloc_self()
actually store the numbers it is using rather than computing them later,
but this fixes symbol lookup after r326203.

Reported by: andreast@
Pointy hat to: me