Platform attribute

This change updates PlatformAttribute to include new OS platforms and add bitness tests. In the end, I think we may abandon PlatformAttribute for something else, but let's get it up-to-date anyway.

The OS changes are simple and straightforward. One small thing: I discovered that we had tests for formats we no longer support, like "Win-6.1". In fact, the 6.1 part is ignored and putting [Win-6.1] on a test would cause an error. I removed those tests and added a test "of the test" to ensure that we only check for acceptable OS names.

The bitness tests are completely new and should be looked at in terms of the interface we want to support. Are we actually interested in process bitness or OS bitness or both? I put in code for both, at least at .NET 4.0 and higher. If we want to support the OS test, I should probably add pInvoke code to test for it, which would be different between Windows and Linux. Since this is an interface issue, I'd like @simoneb and @rprouse to review in particular.