cgo: use slash-free relative paths for .so references

cgo: use slash-free relative paths for .so references

The Makefile and cgo now rewrite / to _ when creating the path.
The .so for gosqlite.googlecode.com/hg/sqlite is named
cgo_gosqlite.googlecode.com_hg_sqlite.so, and then 6l and 8l
both include a default rpath of $GOROOT/pkg/$GOOS_$GOARCH.
This should make it easier to move binaries from one system
to another.

Fixes #857.

R=iant, r
CC=golang-dev
https://golang.org/cl/1700048