container: add riscv64 constants

This target is unlikely to become viable any time soon.

Signed-off-by: Ophestra <cat@gensokyo.uk>

container/std/mksysnum_linux.pl

@@ -12,6 +12,7 @@ my %syscall_cutoff_arch = (
 	"x86" => 340,
 	"x86_64" => 302,
 	"aarch64" => 281,
+	"riscv64" => 281,
 );
 
 print <<EOF;