gocore: work around bad sudog dwarf

Detect when the sudog's elem field type isn't a pointer like it should
be, and overwrite it with a new Type.

Change-Id: Iea9497326358361c688c49c60a33108d434a8d95
Reviewed-on: https://go-review.googlesource.com/98996
Reviewed-by: Keith Randall <khr@golang.org>