Stop importing nanotime from runtime via linkname

[maypok86]

Hi, the authors of Go decided to lock down the use of linkname. Puddle uses linkname only to import nanotime. This PR implements a simple trick that allows puddle to get rid of the use of linkname with almost no impact on performance.

I used this trick when preparing otter v2, as otter needs to store a lot of time.Time and time.Now is often too slow for the otter's needs.

Here are the benchmark results, which show that time.Since is actually equal to nanotime.

goos: darwin
goarch: arm64
pkg: github.com/maypok86/otter/v2/internal/clock
BenchmarkTimeNow
BenchmarkTimeNow-10     	42185438	        34.51 ns/op	       0 B/op	       0 allocs/op
BenchmarkNanotime
BenchmarkNanotime-10    	604727961	         1.920 ns/op	       0 B/op	       0 allocs/op
BenchmarkTimeSince
BenchmarkTimeSince-10       	549684618	         2.055 ns/op	       0 B/op	       0 allocs/op
PASS

[jackc]

LGTM