Necessary to ensure that subsequent tools can continue to find
then end of the export data section simply by searching for "$$".

Adjusted gcimporter used by go/types accordingly.

Also, fixed a bug in gcimporter related to reading export data
in debug format.

Change-Id: Iaea4ed05edd8a5bab28ebe5b19a4740f5e537d35
Reviewed-on: https://go-review.googlesource.com/16283Reviewed-by: Chris Manghane <cmang@golang.org>