libgo/runtime: fix unused-result warning
libgo/runtime: fix unused-result warning

Result of runtime_write is ignored, causing
an unused-result result warning (error in my
case, with -Werror=unused-result).

R=gofrontend-dev, iant
CC=gofrontend-dev
https://golang.org/cl/91690043

qskycolor/gofrontend,qskycolor/gofrontend,golang/gofrontend,golang/gofrontend,anlhord/gofrontend,golang/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,golang/gofrontend,anlhord/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,anlhord/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,anlhord/gofrontend