- In particular:
  1. The alignment requirement imposed by of a non-primitive, non-enum valuetype field is the alignment of that field
  2. The alignment requirement imposed by a primitive is the pointer size of the target platform, unless running on Arm32, in which case if the primitive or enum is 8 bytes in size, the alignment requirement is 8.

- The previous implementation produced an alignment of pointer size, unless running on Arm32 and one of the fields had an alignment requirement of 8 (in which case the alignment requirement computed for the structure would be 8)

In addition, add a test which verifies that the instance field layout test types are actually producing R2R compatible results at runtime.
 - This test shows that we have some issues around explicit layout, so I was forced to disable that portion of the test for now.

Fixes dotnet#65281

…_65281