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/security_windows.go

@@ -372,6 +372,7 @@ type Tokengroups struct {
 //sys	OpenProcessToken(h Handle, access uint32, token *Token) (err error) = advapi32.OpenProcessToken
 //sys	GetTokenInformation(t Token, infoClass uint32, info *byte, infoLen uint32, returnedLen *uint32) (err error) = advapi32.GetTokenInformation
 //sys	GetUserProfileDirectory(t Token, dir *uint16, dirLen *uint32) (err error) = userenv.GetUserProfileDirectoryW
+//sys	getSystemDirectory(dir *uint16, dirLen uint32) (len uint32, err error) = kernel32.GetSystemDirectoryW
 
 // An access token contains the security information for a logon session.
 // The system creates an access token when a user logs on, and every
@@ -468,6 +469,23 @@ func (t Token) GetUserProfileDirectory() (string, error) {
 	}
 }
 
+// GetSystemDirectory retrieves path to current location of the system
+// directory, which is typically, though not always, C:\Windows\System32.
+func GetSystemDirectory() (string, error) {
+	n := uint32(MAX_PATH)
+	for {
+		b := make([]uint16, n)
+		l, e := getSystemDirectory(&b[0], n)
+		if e != nil {
+			return "", e
+		}
+		if l <= n {
+			return UTF16ToString(b[:l]), nil
+		}
+		n = l
+	}
+}
+
 // IsMember reports whether the access token t is a member of the provided SID.
 func (t Token) IsMember(sid *SID) (bool, error) {
 	var b int32