debug/dwarf: add DWARF 4 form constants

Some versions of clang generate DWARF 4-format attributes
even when using -gdwarf-2. We don't care much about the
values, but we do need to be able to parse past them.

This fixes a bug in Go 1.2 rc2 reported via private mail using
a near-tip version of clang.

R=golang-dev, iant, dvyukov
CC=golang-dev
https://codereview.appspot.com/18460043