RISC-V: Align the shadow stack

The standard RISC-V ABIs all require 16-byte stack alignment.  We're
only calling that one function on the shadow stack so I doubt it'd
result in a real issue, but might as well keep this lined up.

Fixes: 31da94c2 ("riscv: add VMAP_STACK overflow detection")
Reviewed-by: Jisheng Zhang <jszhang@kernel.org>
Link: https://lore.kernel.org/r/20221130023515.20217-1-palmer@rivosinc.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>