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>