Prevent GC by volatile [Bug #13150]

	test/ruby/test_marshal.rb test_context_switch (load) and test_gc (dump)
	are failed on FreeBSD 10.3 and gcc7 (FreeBSD Ports Collection) 7.0.0
	20170115 (experimental); RB_GC_GUARD looks not worked well.
	ruby.h: RB_GC_GUARD stronger than gcc7

	* include/ruby/ruby.h (RB_GC_GUARD): prevent guarded pointer from
	  optimization by using as an input to inline asm.
	ruby.h: remove comment

	* include/ruby/ruby.h (RB_GC_GUARD): remove comment unsupported by
	  Solaris AS.
	marshal.c: use hidden objects to allow recycling

	Hidden objects (klass == 0) are not visible to Ruby code invoked
	from other threads or signal handlers, so they can never be
	accessed from other contexts.  This makes it safe to call
	rb_gc_force_recycle on the object slot after releasing malloc
	memory.

	* marshal.c (rb_marshal_dump_limited): hide dump_arg and recycle when done
	  (rb_marshal_load_with_proc): hide load_arg and recycle when done
	  [ruby-core:79518]
	marshal.c: revert r57631 partially

	* marshal.c (rb_marshal_dump_limited): do not free dump_arg, which
	  may be dereferenced in check_dump_arg due to continuation, and
	  get rid of dangling pointers.

	* marshal.c (rb_marshal_load_with_proc): ditto for load_arg.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@57954 b2dd03c8-39d4-4d8f-98ff-823fe69b080e