Many headers in DWARF sections have a "unit length" that can be either
4 bytes or 12 bytes and indicates both the length of the unit and
whether the unit is in 32-bit or 64-bit format.

Currently, we implement unit length parsing in four different places.
Add a "unitLength" method to buf that parses a unit length and use it
in these four places.

Change-Id: I7950b91caaa92aa5e19aa63debc8ae46178ecc4d
Reviewed-on: https://go-review.googlesource.com/7281Reviewed-by: Nigel Tao <nigeltao@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>