syscall: correct comment on where compileCallback is defined

Fixes #11216.

Change-Id: Iad1f4894c2258909484eaf975b08e0f47a82788e
Reviewed-on: https://go-review.googlesource.com/11098Reviewed-by: Andrew Gerrand <adg@golang.org>

src/syscall/syscall_windows.go

@@ -107,7 +107,7 @@ func (e Errno) Timeout() bool {
 	return e == EAGAIN || e == EWOULDBLOCK || e == ETIMEDOUT
 }
 
-// Implemented in asm_windows.s
+// Implemented in runtime/syscall_windows.go.
 func compileCallback(fn interface{}, cleanstack bool) uintptr
 
 // Converts a Go function to a function pointer conforming