eco32: complain on overflow on R_ECO32_W32
eco32: do not fail on unknown instructions

We don't want the disassembly to be abrupted when there's an
instruction we don't know about in the flow, we want the
data word to be printed normally and the rest of the block
disassembled as well.
eco32: apply fixes to pc relative jumps

This implements md_apply_fix for BFD_RELOC_ECO32_METHOD_R16 and
BFD_RELOC_ECO32_METHOD_R26.
It's needed, since bfd (ld) can't handle the relocations to local
symbols correctly.

Note: I'm not sure why ld can't handle them, if that's by design
or by accident. But it doesn't seem to be easily fixed in bfd
and doing the fixups in gas is the "right" way to handle those
relocations anyway.
Also, I'm not certain there are no issues with other relocations
as well, but I only changed the things where there were real
issues for now.