go.sys/windows: grab Syscall interface from the standard syscall package

Also provide stubs to call loadlibrary and getprocaddress, which are implemented
for package syscall by runtime/syscall_windows.goc.
Should get the windows go.sys build closer to green.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://golang.org/cl/126170043

windows/mksyscall_windows.go

@@ -492,10 +492,7 @@ func (f *Fn) SyscallParamCount() int {
 // Syscall determines which SyscallX function to use for function f.
 func (f *Fn) Syscall() string {
 	c := f.SyscallParamCount()
-	if c == 3 {
-		return windowsdot() + "Syscall"
-	}
-	return windowsdot() + "Syscall" + strconv.Itoa(c)
+	return "syscall.Syscall" + strconv.Itoa(c)
 }
 
 // SyscallParamList returns source code for SyscallX parameters for function f.