Make debug-data generation optional

I'm seeing segfaults during linking
when debug data is emitted. I've tried
using LLVM 3.3 and 3.2, both do this.
Thus, I'm adding a -g flag to disable,
but leaving it enabled by default for
now.

llgo-dist still calls llgo-build with
the default, so rebuild packages as
needed with llgo-build.

Also removed the very old debug-related
functions from llgo, and refactored
things slightly.
Merge branch 'master' of github.com:axw/llgo