Switch to use rbconfig for platform detection, allowing support for JRuby

JRuby, Rubinius and MRI all have full support for FFI but RUBY_PLATFORM doesn't do a good job of detecting the host OS (which is what it's actually being used for here) under some of those Ruby implementations.

This commit switches gssapi_loader to use RBConfig's host_os instead which will allow platforms like JRuby to make use of this gem as well.