unix: add ClockSettime on Linux

The syscall clock_settime(3) is used to set the time on the specified
clock. It is a counterpart to clock_gettime(3) which is already
exposed as ClockGettime here.

Fixes golang/go#69830

Change-Id: I6145c915754f31e3c50f6a11568eb9d5ba12a778
GitHub-Last-Rev: 5b15f12
GitHub-Pull-Request: #221
Reviewed-on: https://go-review.googlesource.com/c/sys/+/619255
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: yarikk

unix/syscall_linux.go

@@ -1860,6 +1860,7 @@ func Sendfile(outfd int, infd int, offset *int64, count int) (written int, err e
 //sys	ClockAdjtime(clockid int32, buf *Timex) (state int, err error)
 //sys	ClockGetres(clockid int32, res *Timespec) (err error)
 //sys	ClockGettime(clockid int32, time *Timespec) (err error)
+//sys	ClockSettime(clockid int32, time *Timespec) (err error)
 //sys	ClockNanosleep(clockid int32, flags int, request *Timespec, remain *Timespec) (err error)
 //sys	Close(fd int) (err error)
 //sys	CloseRange(first uint, last uint, flags uint) (err error)