The GNU format does not have a prefix field, so we should make
no attempt to read it. It does however have atime and ctime fields.
Since Go previously placed incorrect values here, we liberally
read the atime and ctime fields and ignore errors so that old tar
files written by Go can at least be partially read.

This fixes half of #12594. The Writer is much harder to fix.

Updates #12594

Change-Id: Ia32845e2f262ee53366cf41dfa935f4d770c7a30
Reviewed-on: https://go-review.googlesource.com/31444
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>