Generate a gap.link script, and use that to generate gap.o. On multi-ABI

Generate a gap.link script, and use that to generate gap.o.  On multi-ABI
architectures we must also link against a gapdummy.o to repair the ABI
of the resulting .o file.  Woe is me.
Also repair install: target to update the linkkit & hash when a kernel
is installed.
ok rpe tb mlarkin and tested by others also