Move gdb support functions to a separate file

Move gdb support functions and variables to
linker_gdb_support.h/cpp

Change-Id: I96c6592a7055715b18f1137367470fe80987263f
diff --git a/linker/debugger.cpp b/linker/debugger.cpp
index 3731c99..b7e84ac 100644
--- a/linker/debugger.cpp
+++ b/linker/debugger.cpp
@@ -27,6 +27,7 @@
  */
 
 #include "linker.h"
+#include "linker_gdb_support.h"
 
 #include <errno.h>
 #include <inttypes.h>