riscv: Increase stack size under KASAN

KASAN requires more stack space because of compiler instrumentation.
Increase stack size as other arches do.

Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Reported-by:  <syzbot+0600986d88e2d4d7ebb8@syzkaller.appspotmail.com>
Fixes: 8ad8b727 ("riscv: Add KASAN support")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>