runtime: get tracking time only when needed

casgstatus currently calls nanotime on every casgstatus when tracking,
even though the time is only used in some cases. For goroutines making
lots of transitions that aren't covered here, this can add a small
overhead. Switch to calling nanotime only when necessary.

Change-Id: I2617869332e8289ef33dd674d786e44dea09aaba
Reviewed-on: https://go-review.googlesource.com/c/go/+/364375


Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>