rts: Fix --debug-numa mode under Docker

As noted in #17606, Docker disallows the get_mempolicy syscall by
default. This caused numerous tests to fail under CI in the `debug_numa`
way. Avoid this by disabling the NUMA probing logic when --debug-numa is
in use, instead setting n_numa_nodes in RtsFlags.c.

rts/Capability.c

@@ -352,6 +352,8 @@ void initCapabilities (void)
         for (i = 0; i < MAX_NUMA_NODES; i++) {
             numa_map[i] = 0;
         }
+    } else if (RtsFlags.DebugFlags.numa) {
+        // n_numa_nodes was set by RtsFlags.c
     } else {
         uint32_t nNodes = osNumaNodes();
         if (nNodes > MAX_NUMA_NODES) {

rts/RtsFlags.c

@@ -985,6 +985,7 @@ error = true;
                           RtsFlags.GcFlags.numa = true;
                           RtsFlags.DebugFlags.numa = true;
                           RtsFlags.GcFlags.numaMask = (1<<nNodes) - 1;
+                          n_numa_nodes = nNodes;
                       }
                   }
 #endif