debug: Print information about channels.  Add ChanType to DWARF parser.

Channels were previously printed as raw pointers.

Change-Id: I4b2b4e4e91f2aabd08ddda0f29ccdee5a91ce288
Reviewed-on: https://go-review.googlesource.com/2717
Reviewed-by: Nigel Tao <nigeltao@golang.org>