swift234

Add support for __gnuc_va_list to ClangImporter

The implementation of va_list is pretty inconsistent across platforms.
On linux-arm __gnuc_va_list is the type used. This change adds that to ClangImporter.
The change to the test is optional, but it dramatically improves the error message printed to the console when the test fails.
Finally, the change to the application of alignment padding is due to the fact that all ARM platforms other than iOS need AAPCS-complient alignemnt.