runtime: Don't use filename without '/' for backtrace library.
runtime: Don't use filename without '/' for backtrace library.

Fixes http://golang.org/issue/6715.

R=iant
CC=gofrontend-dev
https://golang.org/cl/26440044

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