core: add SystemCallArchitectures= unit setting to allow disabling of non-native

architecture support for system calls

Also, turn system call filter bus properties into complex types instead
of concatenated strings.

src/core/execute.h

@@ -168,6 +168,7 @@ struct ExecContext {
         bool same_pgrp;
 
         Set *syscall_filter;
+        Set *syscall_archs;
         int syscall_errno;
         bool syscall_whitelist:1;