[RISCV] Add a test showing incorrect RVV stack alignment

The RISC-V stack is assumed to be aligned to 16 bytes and can handle stack
realignment for larger objects, but the "RVV stack" is only ensured to be
aligned to 8 bytes. This means that objects specified at a larger alignment may
be misaligned, not only for 16-byte-aligned RVV objects that don't trigger
realignment, but also for 32-byte-and-larger-aligned objects which do.

The new test checks a variety of alignment configurations, showing the
misaligned cases.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D110933