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

src/platform-solaris.cc

Index: src/platform-solaris.cc
diff --git a/src/platform-solaris.cc b/src/platform-solaris.cc
index df81c3a6bc87fc82042eb45138afeafe570f19d0..a0590cbecb89ed78507ac8c81c3099a32868c11e 100644
--- a/src/platform-solaris.cc
+++ b/src/platform-solaris.cc
@@ -51,7 +51,6 @@
 
 #include "v8.h"
 
-#include "platform-posix.h"
 #include "platform.h"
 #include "v8threads.h"
 #include "vm-state-inl.h"
@@ -112,11 +111,6 @@ 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)