fix Dwarf2 build with certain gcc versions

Older gcc reports:

.../bfd/dwarf2.c: In function 'read_ranges':
.../bfd/dwarf2.c:3107: error: comparison between signed and unsigned
.../bfd/dwarf2.c: In function 'read_rnglists':
.../bfd/dwarf2.c:3189: error: comparison between signed and unsigned

Similarly for binutils/dwarf.c. Arrange for the left sides of the > to
also be unsigned quantities.