debug/elf: fix offset for GNU version symbols
debug/elf: fix offset for GNU version symbols

Since we no longer skip the first entry when reading a symbol table,
we no longer need to allow for the offset difference when processing
the GNU version symbols.

Unbreaks builds on Linux.

R=golang-dev, agl, iant
CC=golang-dev
http://codereview.appspot.com/6843057
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go