Fix StructPacking test for ARM (#16484)

Fix StructPacking test for ARM (#16484)

Use RuntimeInformation.ProcessArchitecture instead of
RuntimeInformation.OSArchitecture, as for Windows ARM we normally
run in the WoW layer.

Also, `double` on ARM is 8 byte aligned, but was going down the x86
code path that assumes 4-byte alignment.

Finally, add this test to the Windows ARM and Windows ARM64 test
runs by adding it to the respective tests.lst files.