Pass and remap `matrix_architecture` to KeyDB role

Only `amd64` and `arm64` actually work.

The KeyDB role includes a validation task and will complain about
unsupported architectures (like `arm32`).

`arm32` users can stick to Redis for now (`keydb_enabled: false` + `redis_enabled: true`) until:
- the KeyDB role starts supporting self-building.. although building such large
  projects on weak CPUs is probably impractical
- a prebuilt arm32 image is made available by other means

group_vars/matrix_servers

@@ -3992,6 +3992,15 @@ keydb_gid: "{{ matrix_user_gid }}"
 
 keydb_base_path: "{{ matrix_base_data_path }}/keydb"
 
+keydb_arch: |-
+  {{
+    ({
+      'amd64': 'x86_64',
+      'arm32': 'arm32',
+      'arm64': 'arm64',
+    })[matrix_architecture]
+  }}
+
 ######################################################################
 #
 # keydb