* dwarf/line: implement DW_LNE_set_discriminator

We don't use the discriminator field in any way but we need to at least
parse it to support debub_line programs that use it.

* dwarf/line: support parsing DWARF4 debug_line sections

There is an extra field maximum_operations_per_instruction that is used
for VLIW CPUs. We don't support this feature but we have to at least
parse the field to not crash.