cmd/gc: update hashmap code location in comments

Change-Id: I3b81f2e9eb29ee6349d758b68fe7951b34f15a81
Reviewed-on: https://go-review.googlesource.com/1974
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
runtime: hashmap: move overflow pointer to end of bucket

Pointers to zero-sized values may end up pointing to the next
object in memory, and possibly off the end of a span.  This
can cause memory leaks and/or confuse the garbage collector.

By putting the overflow pointer at the end of the bucket, we
make sure that pointers to any zero-sized keys or values don't
accidentally point to the next object in memory.

fixes #9384

Change-Id: I5d434df176984cb0210b4d0195dd106d6eb28f73
Reviewed-on: https://go-review.googlesource.com/1869
Reviewed-by: Russ Cox <rsc@golang.org>