libgo: fix go_export extraction on Darwin

On Darwin, the section name is prefixed with the segment name, __GNU_GO.

Change-Id: I395a47d9922248ad19bda668946decb62435d3d8
Reviewed-on: https://go-review.googlesource.com/c/151097
Reviewed-by: Ian Lance Taylor <iant@golang.org>
2 files changed