runtime: protect plan9 time_now function with !faketime build tag
The introduction of monotonic time support for Plan 9 in CL 656755
causes a build error with multiple declaration of time_now when
built with tag faketime. Correct this by moving function time_now
into its own source file with !faketime build tag.
Fixes #73169
Change-Id: Id7a9a1c77e286511e25546089681f2f88a9538bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/662856 Reviewed-by: Austin Clements <austin@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com> Reviewed-by: David du Colombier <0intro@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>