windows: use proper system directory path in fallback loader

The %WINDIR% variable is an odd choice and not even entirely reliable.
Since Windows 2000, there has been a specific function for determining
this information, so let's use it. It's also a useful function in its
own right for folks who want to launch system tools in a somewhat safe
way, like netsh.exe.

Updates golang/go#14959
Updates golang/go#30642

Change-Id: Ic24baf37d14f2daced0c1db2771b5a673d2c8852
Reviewed-on: https://go-review.googlesource.com/c/sys/+/165759
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

windows/zsyscall_windows.go

@@ -252,6 +252,7 @@ var (
 	procOpenProcessToken                   = modadvapi32.NewProc("OpenProcessToken")
 	procGetTokenInformation                = modadvapi32.NewProc("GetTokenInformation")
 	procGetUserProfileDirectoryW           = moduserenv.NewProc("GetUserProfileDirectoryW")
+	procGetSystemDirectoryW                = modkernel32.NewProc("GetSystemDirectoryW")
 )
 
 func RegisterEventSource(uncServerName *uint16, sourceName *uint16) (handle Handle, err error) {
@@ -2718,3 +2719,16 @@ func GetUserProfileDirectory(t Token, dir *uint16, dirLen *uint32) (err error) {
 	}
 	return
 }
+
+func getSystemDirectory(dir *uint16, dirLen uint32) (len uint32, err error) {
+	r0, _, e1 := syscall.Syscall(procGetSystemDirectoryW.Addr(), 2, uintptr(unsafe.Pointer(dir)), uintptr(dirLen), 0)
+	len = uint32(r0)
+	if len == 0 {
+		if e1 != 0 {
+			err = errnoErr(e1)
+		} else {
+			err = syscall.EINVAL
+		}
+	}
+	return
+}