Fix marshaling with gcc7. Based on upstream revision 57410:
https://svn.ruby-lang.org/cgi-bin/viewvc.cgi?view=revision&revision=57410
https://github.com/ruby/ruby/commit/7c1b30a602ab109d8d5388d7dfb3c5b180ba24e1
https://bugs.ruby-lang.org/issues/13150

with the upstream patches intent ported to Ruby 2.2.5

Upstream-Status: Backport

Signed-off-by: Joshua Lock <joshua.g.lock@intel.com>

Index: ruby-2.2.5/marshal.c
===================================================================
--- ruby-2.2.5.orig/marshal.c
+++ ruby-2.2.5/marshal.c
@@ -17,7 +17,6 @@
 #include "ruby/io.h"
 #include "ruby/st.h"
 #include "ruby/util.h"
-
 #include <math.h>
 #ifdef HAVE_FLOAT_H
 #include <float.h>
@@ -985,7 +984,7 @@ marshal_dump(int argc, VALUE *argv)
     VALUE obj, port, a1, a2;
     int limit = -1;
     struct dump_arg *arg;
-    VALUE wrapper; /* used to avoid memory leak in case of exception */
+    volatile VALUE wrapper; /* used to avoid memory leak in case of exception */
 
     port = Qnil;
     rb_scan_args(argc, argv, "12", &obj, &a1, &a2);