Extend the allowed bitfield range (ie. that for which

Extend the allowed bitfield range (ie. that for which
complain_overflow_bitfield doesn't complain) from -2**(n-1)..2**n-1 to
-2**n..2**n.  This might mean that some reloc overflows are no longer
caught, but it solves the address wrap problem for 16-bit relocs
nicely.  In any case, ports that rely on complain_overflow_bitfield
for reloc overflow checking were not getting a very good check
previously.  A bitfield range in a machine instruction is typically
either the signed or unsigned n bit numbers, not the overlap of these
two ranges.