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

src/platform-freebsd.cc

Index: src/platform-freebsd.cc
diff --git a/src/platform-freebsd.cc b/src/platform-freebsd.cc
index 518ad3185f01c026fcf358261b7ad3fb964b871f..103fd6ce054e6fe17cfbff9fbf97e38567a27ab9 100644
--- a/src/platform-freebsd.cc
+++ b/src/platform-freebsd.cc
@@ -43,7 +43,6 @@
 #include <sys/fcntl.h>  // open
 #include <unistd.h>     // getpagesize
 // If you don't have execinfo.h then you need devel/libexecinfo from ports.
-#include <execinfo.h>   // backtrace, backtrace_symbols
 #include <strings.h>    // index
 #include <errno.h>
 #include <stdarg.h>
@@ -54,7 +53,6 @@
 #include "v8.h"
 #include "v8threads.h"
 
-#include "platform-posix.h"
 #include "platform.h"
 #include "vm-state-inl.h"
 
@@ -97,11 +95,6 @@ void* OS::Allocate(const size_t requested,
 }
 
 
-void OS::DumpBacktrace() {
-  POSIXBacktraceHelper<backtrace, backtrace_symbols>::DumpBacktrace();
-}
-
-
 class PosixMemoryMappedFile : public OS::MemoryMappedFile {
  public:
   PosixMemoryMappedFile(FILE* file, void* memory, int size)