[ob] fix buf size in amd64_epilogue_frame_cache

[Michael Snyder]

Joel Brobecker wrote:
>> 2009-12-20  Michael Snyder  <msnyder@vmware.com>
>>
>> 	* amd64-tdep.c (amd64_epilogue_frame_cache): Fix buf size.
> 
> I'm going to apply this patch to the gdb-7.0 branch as well.
> 
>> -  gdb_byte buf[4];
>> +  gdb_byte buf[8];
> 
> I think we should try to avoid using litteral constants like these,
> especially when we repeat that constant later.  For instance, in
> this case, we could have used register_size, or something similar...

I don't think there's a simple "register_size" constant.
There's a register_size function, but that's not a good way
to declare an array size.

There's MAX_REGISTER_SIZE, but that's 16.  A possibility.
If you've got a better suggestion...