align jmp_buf to VALUE

This ec->machine.regs is marked by GC. However jmp_buf is
not defined by us.  There are chances of unaligned access.
We should force it VALUE-aligned.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63375 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

vm_core.h

@@ -823,7 +823,7 @@ typedef struct rb_execution_context_struct {
 	VALUE *register_stack_end;
 	size_t register_stack_maxsize;
 #endif
-	jmp_buf regs;
+	RUBY_ALIGNAS(SIZEOF_VALUE) jmp_buf regs;
     } machine;
 } rb_execution_context_t;