Revert "[#1262] sdnotify: Get rid of go:linkname for nanotime"

This reverts commit 327d364f34.

Reverted due to the problem with reload signal sent by systemd.
`frostfs-ir` service reconfigures correctly and service's
statuses are being reported to systemd. However, since we
replaced `go:linkname` & `nanotime()` with `time.Since()`,
systemd refuses to accept reload signal response from
`frostfs-ir`. To maintain correct behaviour it was decided to
revevrt systemd-related changes until a better solution is
found.

Signed-off-by: Ekaterina Lebedeva <ekaterina.lebedeva@yadro.com>

pkg/util/sdnotify/clock.go

@@ -0,0 +1,10 @@
+package sdnotify
+
+import (
+	// For go:linkname to work.
+	_ "unsafe"
+)
+
+//go:noescape
+//go:linkname nanotime runtime.nanotime
+func nanotime() int64

pkg/util/sdnotify/clock.s

@@ -0,0 +1,2 @@
+// The file is intentionally empty.
+// It is a workaround for https://github.com/golang/go/issues/15006

pkg/util/sdnotify/sdnotify.go

@@ -6,7 +6,6 @@ import (
 	"net"
 	"os"
 	"strings"
-	"time"
 )
 
 const (
@@ -17,7 +16,6 @@ const (
 
 var (
 	socket *net.UnixAddr
-	start  = time.Now()
 
 	errSocketVariableIsNotPresent = errors.New("\"NOTIFY_SOCKET\" environment variable is not present")
 	errSocketIsNotInitialized     = errors.New("socket is not initialized")
@@ -53,7 +51,7 @@ func FlagAndStatus(status string) error {
 		//    must be sent, containing "READY=1".
 		//
 		// For MONOTONIC_USEC format refer to https://www.man7.org/linux/man-pages/man3/sd_notify.3.html
-		status += fmt.Sprintf("\nMONOTONIC_USEC=%d", uint64(time.Since(start))/1e3 /* microseconds in nanoseconds */)
+		status += fmt.Sprintf("\nMONOTONIC_USEC=%d", uint64(nanotime())/1e3 /* microseconds in nanoseconds */)
 	}
 	status += "\nSTATUS=" + strings.TrimSuffix(status, "=1")
 	return Send(status)