riscv: Bump COMMAND_LINE_SIZE value to 1024

Increase COMMAND_LINE_SIZE as the current default value is too low
for syzbot kernel command line.

There has been considerable discussion on this patch that has led to a
larger patch set removing COMMAND_LINE_SIZE from the uapi headers on all
ports.  That's not quite done yet, but it's gotten far enough we're
confident this is not a uABI change so this is safe.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Link: https://lore.kernel.org/r/20210316193420.904-1-alex@ghiti.fr
[Palmer: it's not uabi]
Link: https://lore.kernel.org/linux-riscv/874b8076-b0d1-4aaa-bcd8-05d523060152@app.fastmail.com/#t
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>