windows: add accessor for duration since boot

In order to deal with boot time race conditions, sometimes it is
useful to determine the time since boot.

Change-Id: Ibc907b49a9b072b3ef3b6c94eec7e2e6428943ba
Reviewed-on: https://go-review.googlesource.com/c/sys/+/180899
Run-TryBot: Jason Donenfeld <Jason@zx2c4.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>

windows/syscall_windows.go

@@ -10,6 +10,7 @@ import (
 	errorspkg "errors"
 	"sync"
 	"syscall"
+	"time"
 	"unicode/utf16"
 	"unsafe"
 )
@@ -194,6 +195,7 @@ func NewCallbackCDecl(fn interface{}) uintptr {
 //sys	SetEnvironmentVariable(name *uint16, value *uint16) (err error) = kernel32.SetEnvironmentVariableW
 //sys	CreateEnvironmentBlock(block **uint16, token Token, inheritExisting bool) (err error) = userenv.CreateEnvironmentBlock
 //sys	DestroyEnvironmentBlock(block *uint16) (err error) = userenv.DestroyEnvironmentBlock
+//sys	getTickCount64() (ms uint64) = kernel32.GetTickCount64
 //sys	SetFileTime(handle Handle, ctime *Filetime, atime *Filetime, wtime *Filetime) (err error)
 //sys	GetFileAttributes(name *uint16) (attrs uint32, err error) [failretval==INVALID_FILE_ATTRIBUTES] = kernel32.GetFileAttributesW
 //sys	SetFileAttributes(name *uint16, attrs uint32) (err error) = kernel32.SetFileAttributesW
@@ -498,6 +500,10 @@ func ComputerName() (name string, err error) {
 	return string(utf16.Decode(b[0:n])), nil
 }
 
+func DurationSinceBoot() time.Duration {
+	return time.Duration(getTickCount64()) * time.Millisecond
+}
+
 func Ftruncate(fd Handle, length int64) (err error) {
 	curoffset, e := Seek(fd, 0, 1)
 	if e != nil {