Add the ability to specify the list of extra import paths
Overhaul how export data works

This makes the following changes to export data handling:

1) Updates us to the latest version of the following CL:
   https://codereview.appspot.com/78740043

2) Append init data (i.e. priorities) to condensed export data.
   This is the format expected by the importer in the above CL, and
   (tentatively) the format that gccgo will use for condensed export data.

3) Modify the compiler to return export data instead of writing
   it to disk. The main motivation for this is to allow clients to write
   export data in an appropriate place. For example, a client which uses
   LTO may want to write export files separately, while a client that emits
   object files may want to store the export data in a separate section
   in the object file.
Merge pull request #138 from pcc/libgo-p1

Overhaul how export data works