Fix cgo for GCC 4.4

Firstly, with -Werror, GCC switched to printing warnings starting
with "error:". Widening the string matches solves this as the messages
are otherwise unchanged.

Secondly, GCC 4.4 outputs DWARF sections with with NUL bytes in all
the offsets and requires the relocation section for .debug_info to be
processed in order to result in valid DWARF data. Thus we add minimal
handling for relocation sections, which is sufficient for our needs.

BUG=1
Fixes #1.

R=rsc, iant
CC=go-dev
http://go/go-review/1017003