Skip 64 bit MWMessagePack tests on 32 bit machines

Skip 64 bit MWMessagePack tests on 32 bit machines

It's not only Windows. I have an Ubuntu machine with a PHP setup
limited to 32 bit (which is totally fine, it allows me to develop
and always stay compatible to Windows and other 32 bit environments).

I don't see an other way than to skip these tests on a 32 bit
machine. The loss (PHP clips values larger than 2147483647 to
2147483647) does not happen in the code that is tested but in the
test setup.

Change-Id: Ie9a173c0a92ed84eaaea981a25ba130f2eca169e