cmd/go: do not fail if libgcc does not exist
cmd/go: do not fail if libgcc does not exist

(This is a patch from the pkgsrc Go package.)

LGTM=iant
R=golang-codereviews, iant, joerg.sonnenberger, dave
CC=golang-codereviews, joerg
https://codereview.appspot.com/108340043

Committer: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,wesolows/golang-old