riscv, bpf: Use STACK_ALIGN macro for size rounding up

Use the macro STACK_ALIGN that is defined in asm/processor.h for stack size
rounding up, just like bpf_jit_comp32.c does.

Signed-off-by: Xiao Wang <xiao.w.wang@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Pu Lehui <pulehui@huawei.com>
Link: https://lore.kernel.org/bpf/20240523031835.3977713-1-xiao.w.wang@intel.com