AVR indirect_jump addresses limited to 16 bits

[Paul Schlie]

> From: Paul Brook <paul@codesourcery.com>
> Don't we know which labels are targets of indirect jumps?
> So the proposed restriction now becomes: functions *and targets of indirect
> jumps* must be aligned to an N word boundary. I'd guess that the latter are
> sufficiently rare that this is still an acceptable restriction.

- seams plausible, if it were reasonably easy to identify such labels to the
linker with a corresponding alignment requirement to the exclusion of all
other labels? (and the restricted alignment method were deemed acceptable?)