cmd/link: writelines should keep is_stmt in sync with what it's writing

For all functions but the last one if the function ends on a
non-statement instruction the statement flag in debug_line is changed
but is_stmt is not updated to match.

Change-Id: I03c275c5e261ea672ce4da7baca2458810708326
Reviewed-on: https://go-review.googlesource.com/112979


Reviewed-by: David Chase <drchase@google.com>