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

src/platform-openbsd.cc

Index: src/platform-openbsd.cc
diff --git a/src/platform-openbsd.cc b/src/platform-openbsd.cc
index 4f5420ee45c97e5ede7ee0cde34118f1aac7c947..710c3904afbe49ddee51e42f45e515ca6e63e4f5 100644
--- a/src/platform-openbsd.cc
+++ b/src/platform-openbsd.cc
@@ -42,7 +42,6 @@
 #include <sys/stat.h>   // open
 #include <fcntl.h>      // open
 #include <unistd.h>     // sysconf
-#include <execinfo.h>   // backtrace, backtrace_symbols
 #include <strings.h>    // index
 #include <errno.h>
 #include <stdarg.h>
@@ -51,7 +50,6 @@
 
 #include "v8.h"
 
-#include "platform-posix.h"
 #include "platform.h"
 #include "v8threads.h"
 #include "vm-state-inl.h"
@@ -96,11 +94,6 @@ 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)