runtime: convert NewCallback and NewCallbackCDecl to Go

LGTM=khr
R=khr, remyoudompheng
CC=golang-codereviews
https://golang.org/cl/132820043
diff --git a/src/pkg/runtime/syscall_windows.go b/src/pkg/runtime/syscall_windows.go
new file mode 100644
index 0000000..272db62
--- /dev/null
+++ b/src/pkg/runtime/syscall_windows.go
@@ -0,0 +1,92 @@
+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package runtime
+
+import (
+	"unsafe"
+)
+
+type callbacks struct {
+	lock
+	ctxt [cb_max]*wincallbackcontext
+	n    int
+}
+
+func (c *wincallbackcontext) isCleanstack() bool {
+	return c.cleanstack == 1
+}
+
+func (c *wincallbackcontext) setCleanstack(cleanstack bool) {
+	if cleanstack {
+		c.cleanstack = 1
+	} else {
+		c.cleanstack = 0
+	}
+}
+
+var (
+	cbs     callbacks
+	cbctxts **wincallbackcontext = &cbs.ctxt[0] // to simplify access to cbs.ctxt in sys_windows_*.s
+
+	callbackasm byte // type isn't really byte, it's code in runtime
+)
+
+// callbackasmAddr returns address of runtime.callbackasm
+// function adjusted by i.
+// runtime.callbackasm is just a series of CALL instructions
+// (each is 5 bytes long), and we want callback to arrive at
+// correspondent call instruction instead of start of
+// runtime.callbackasm.
+func callbackasmAddr(i int) uintptr {
+	return uintptr(add(unsafe.Pointer(&callbackasm), uintptr(i*5)))
+}
+
+func compileCallback(fn eface, cleanstack bool) (code uintptr) {
+	if fn._type == nil || (fn._type.kind&kindMask) != kindFunc {
+		panic("compilecallback: not a function")
+	}
+	ft := (*functype)(unsafe.Pointer(fn._type))
+	if len(ft.out) != 1 {
+		panic("compilecallback: function must have one output parameter")
+	}
+	uintptrSize := uint(unsafe.Sizeof(uintptr(0)))
+	if t := (**_type)(unsafe.Pointer(&ft.out[0])); (*t).size != uintptrSize {
+		panic("compilecallback: output parameter size is wrong")
+	}
+	argsize := uint(0)
+	for _, t := range (*[1024](*_type))(unsafe.Pointer(&ft.in[0]))[:len(ft.in)] {
+		if (*t).size != uintptrSize {
+			panic("compilecallback: input parameter size is wrong")
+		}
+		argsize += uintptrSize
+	}
+
+	golock(&cbs.lock)
+	defer gounlock(&cbs.lock)
+
+	n := cbs.n
+	for i := 0; i < n; i++ {
+		if cbs.ctxt[i].gobody == fn.data && cbs.ctxt[i].isCleanstack() == cleanstack {
+			return callbackasmAddr(i)
+		}
+	}
+	if n >= cb_max {
+		gothrow("too many callback functions")
+	}
+
+	c := new(wincallbackcontext)
+	c.gobody = fn.data
+	c.argsize = argsize
+	c.setCleanstack(cleanstack)
+	if cleanstack && argsize != 0 {
+		c.restorestack = argsize
+	} else {
+		c.restorestack = 0
+	}
+	cbs.ctxt[n] = c
+	cbs.n++
+
+	return callbackasmAddr(n)
+}