riscv: Match memory barriers between send_ipi_many and handle_ipi

riscv: Match memory barriers between send_ipi_many and handle_ipi

Without a matching barrier on the write side, the barrier in handle_ipi
does nothing. It was entirely possible for the boot hart to write to addr,
arg0, and arg1 *after* sending the IPI, because there was no barrier on the
sending side.

Fixes: 90ae281437 ("riscv: add option to wait for ack from secondary harts in smp functions")
Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Rick Chen <rick@andestech.com>
Reviewed-by: Leo Liang <ycliang@andestech.com>