runtime: change to implement monotonic time in Plan 9 breaks TestFakeTime on plan9 builders

[millerresearch]

Following the merge of CL 656755, runtime test always fails on Plan 9 builders:

=== RUN   TestFakeTime
=== PAUSE TestFakeTime
=== CONT  TestFakeTime
    crash_test.go:173: running /home/swarming/.swarming/w/ir/x/w/goroot/bin/go build -o /tmp/go-build1685094314/testfaketime_-tags=faketime.exe -tags=faketime
    time_test.go:32: building testfaketime [-tags=faketime]: exit status: 'go 333592: 1'
        # runtime
        ../../time_fake.go:41:6: time_now redeclared in this block
        	../../os_plan9.go:600:6: other declaration of time_now
        
--- FAIL: TestFakeTime (48.27s)

I believe the problem is that the time_now function in runtime/os_plan9.go needs to be in its own file with a !faketime build tag.

[gopherbot]

Change https://go.dev/cl/662856 mentions this issue: runtime: protect plan9 time_now function with !faketime build tag