riscv: Randomize lower bits of stack address

Implement arch_align_stack() to randomize the lower bits
of the stack address.

Signed-off-by: Yunhui Cui <cuiyunhui@bytedance.com>
Link: https://lore.kernel.org/r/20240625030502.68988-1-cuiyunhui@bytedance.com


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