riscv: return -ENOSYS for syscall -1

Properly return -ENOSYS for syscall -1 instead of leaving the return value
uninitialized.  This fixes the strace teststuite.

Fixes: 5340627e

 ("riscv: add support for SECCOMP and SECCOMP_FILTER")
Cc: stable@vger.kernel.org
Signed-off-by: Andreas Schwab <schwab@suse.de>
Reviewed-by: Tycho Andersen <tycho@tycho.pizza>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>