[RISCV] Fix RVV stack frame alignment bugs

This patch addresses several alignment issues in the stack frame when
RVV objects are taken into account.

One bug is that the RVV stack was never guaranteed to keep the alignment
of the stack *as a whole*. We must maintain a 16-byte aligned stack at
all times, especially when calling other functions. With the standard V
extension, this is conveniently happening since VLEN is at least 128 and
always 16-byte aligned. However, we support Zvl64b which does not
guarantee this. To fix this, the RVV stack size is rounded up to be
aligned to 16 bytes. This in practice generally makes us allocate a
stack sized at least 2*VLEN in size, and a multiple of 2.

    |------------------------------| -- <-- FP
    | 8-byte callee-save           | |      |
    |------------------------------| |      |
    | one VLENB-sized RVV object   | |      |
    |------------------------------| |      |
    | 8-byte local variable        | |      |
    |------------------------------| -- <--...