Move DumpBacktrace() to checks.cc and cleanup both the code and the necessary platform checks.

src/platform-cygwin.cc

Index: src/platform-cygwin.cc
diff --git a/src/platform-cygwin.cc b/src/platform-cygwin.cc
index 5903438932b61ad6eea29c7cac5441bca78c632a..0076d567f8b29711f881c6da6dffdfd8023325fb 100644
--- a/src/platform-cygwin.cc
+++ b/src/platform-cygwin.cc
@@ -41,7 +41,6 @@
 
 #include "v8.h"
 
-#include "platform-posix.h"
 #include "platform.h"
 #include "simulator.h"
 #include "v8threads.h"
@@ -88,11 +87,6 @@ void* OS::Allocate(const size_t requested,
 }
 
 
-void OS::DumpBacktrace() {
-  // Currently unsupported.
-}
-
-
 class PosixMemoryMappedFile : public OS::MemoryMappedFile {
  public:
   PosixMemoryMappedFile(FILE* file, void* memory, int size)