tstime/mono: remove unsafe

This removes the unsafe/linkname and only uses the standard library.

It's a bit slower, for now, but https://go.dev/cl/518336 should get us
back.

On darwin/arm64, without https://go.dev/cl/518336

    pkg: tailscale.com/tstime/mono
              │   before    │                after                │
              │   sec/op    │   sec/op     vs base                │
    MonoNow-8   16.20n ± 0%   19.75n ± 0%  +21.92% (p=0.000 n=10)
    TimeNow-8   39.46n ± 0%   39.40n ± 0%   -0.16% (p=0.002 n=10)
    geomean     25.28n        27.89n       +10.33%

And with it,

    MonoNow-8   16.34n ±  1%   16.93n ± 0%  +3.67% (p=0.001 n=10)
    TimeNow-8   39.55n ± 15%   38.46n ± 1%  -2.76% (p=0.000 n=10)
    geomean     25.42n         25.52n       +0.41%

Updates #8839
Updates tailscale/go#70

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstime/mono/mono.go

@@ -16,7 +16,6 @@ import (
 	"fmt"
 	"sync/atomic"
 	"time"
-	_ "unsafe" // for go:linkname
 )
 
 // Time is the number of nanoseconds elapsed since an unspecified reference start time.
@@ -29,7 +28,7 @@ func Now() Time {
 	// The corresponding package time expression never does, if the wall clock is correct.
 	// Preserve this correspondence by increasing the "base" monotonic clock by a fair amount.
 	const baseOffset int64 = 1 << 55 // approximately 10,000 hours in nanoseconds
-	return Time(now() + baseOffset)
+	return Time(int64(time.Since(baseWall)) + baseOffset)
 }
 
 // Since returns the time elapsed since t.
@@ -72,9 +71,6 @@ func (t *Time) LoadAtomic() Time {
 	return Time(atomic.LoadInt64((*int64)(t)))
 }
 
-//go:linkname now runtime.nanotime1
-func now() int64
-
 // baseWall and baseMono are a pair of almost-identical times used to correlate a Time with a wall time.
 var (
 	baseWall time.Time