Currently we report MemStats.PauseEnd in nanoseconds, but with no
particular 0 time. On Linux, the 0 time is when the host started. On
Darwin, it's the UNIX epoch. This is also inconsistent with the other
absolute time in MemStats, LastGC, which is always reported in
nanoseconds since 1970.

Fix PauseEnd so it's always reported in nanoseconds since 1970, like
LastGC.

Fixes one of the issues raised in #10323.

Change-Id: Ie2fe3169d45113992363a03b764f4e6c47e5c6a8
Reviewed-on: https://go-review.googlesource.com/11801
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>