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>
diff --git a/windows/zsyscall_windows.go b/windows/zsyscall_windows.go
index dcd10e0..a7663b5 100644
--- a/windows/zsyscall_windows.go
+++ b/windows/zsyscall_windows.go
@@ -133,6 +133,7 @@
 	procSetEnvironmentVariableW            = modkernel32.NewProc("SetEnvironmentVariableW")
 	procCreateEnvironmentBlock             = moduserenv.NewProc("CreateEnvironmentBlock")
 	procDestroyEnvironmentBlock            = moduserenv.NewProc("DestroyEnvironmentBlock")
+	procGetTickCount64                     = modkernel32.NewProc("GetTickCount64")
 	procSetFileTime                        = modkernel32.NewProc("SetFileTime")
 	procGetFileAttributesW                 = modkernel32.NewProc("GetFileAttributesW")
 	procSetFileAttributesW                 = modkernel32.NewProc("SetFileAttributesW")
@@ -1373,6 +1374,12 @@
 	return
 }
 
+func getTickCount64() (ms uint64) {
+	r0, _, _ := syscall.Syscall(procGetTickCount64.Addr(), 0, 0, 0, 0)
+	ms = uint64(r0)
+	return
+}
+
 func SetFileTime(handle Handle, ctime *Filetime, atime *Filetime, wtime *Filetime) (err error) {
 	r1, _, e1 := syscall.Syscall6(procSetFileTime.Addr(), 4, uintptr(handle), uintptr(unsafe.Pointer(ctime)), uintptr(unsafe.Pointer(atime)), uintptr(unsafe.Pointer(wtime)), 0, 0)
 	if r1 == 0 {