runtime: rename _defer struct _panic field to panicStack

The gc version of the _defer struct has a _panic field that has a
completely different meaning. We are going to want that bring that new
meaning into the gofrontend to improve panic reports with nested
panic calls. Simplify that by first renaming the existing _panic field.

Change-Id: I9ae03cacaa501c72de0d049e9bf36da5b736423c
Reviewed-on: https://go-review.googlesource.com/46454
Reviewed-by: Than McIntosh <thanm@google.com>
2 files changed