Fix ETW package to work on 386

The ETW registration handle is defined to be 64-bits on all platforms. The Go
type previously used a uintptr which created problems on 32-bit systems. This
change adds a new wrapper file which receives a 64-bit handle for the functions
defined in it, and correctly passes it to the native ETW functions either as-is
(on 64-bit) or as two 32-bit values (on 32-bit).

This required a minor change in mksyscall_windows.go, to allow multiple syscalls
that map to the same underlying function without causing a duplicate definition
error in the generated file.

pkg/etw/mksyscall_windows.go

@@ -603,6 +603,19 @@ type Source struct {
 	ExternalImports []string
 }
 
+func (src *Source) FuncImports() []*Fn {
+	uniq := make(map[string]bool)
+	r := make([]*Fn, 0)
+	for _, f := range src.Funcs {
+		name := f.DLLFuncName()
+		if _, found := uniq[name]; !found {
+			uniq[name] = true
+			r = append(r, f)
+		}
+	}
+	return r
+}
+
 func (src *Source) Import(pkg string) {
 	src.StdLibImports = append(src.StdLibImports, pkg)
 	sort.Strings(src.StdLibImports)
@@ -865,7 +878,7 @@ var (
 {{define "dlls"}}{{range .DLLs}}	mod{{.}} = {{newlazydll .}}
 {{end}}{{end}}
 
-{{define "funcnames"}}{{range .Funcs}}	proc{{.DLLFuncName}} = mod{{.DLLName}}.NewProc("{{.DLLFuncName}}")
+{{define "funcnames"}}{{range .FuncImports}}	proc{{.DLLFuncName}} = mod{{.DLLName}}.NewProc("{{.DLLFuncName}}")
 {{end}}{{end}}
 
 {{define "helperbody"}}