Change IA64 to ia64 in architecture.rb for HP-UX Itanium, update and improve spec test.

I wrongly thought that HP-UX Itanium would report uname -m as "IA64", in fact it's "ia64" and this is what Ohai puts in node['kernel']['machine']. So I've corrected the code in architecture.rb and updated the corresponding spec test. I've also extended the spec test for _32_bit? to check for false for all the 64 bit architectures coded.

My apologies for the inconvenience.