Use a better arbitrary target arch for this host-only "device".

This "device" is used for linux_bionic builds that may build with
prebuilts (from prebuilts/runtime/mainline and prebuilts/module_sdk) on
master-art and master-art-host. The ndk arch suite was recently
extended to riscv64 (https://r.android.com/2488597), and those
prebuilts don't include that arch, which caused a failure in Soong
(even though target prebuilts aren't actually used here).

Test: lunch armv8-eng
      env DIST_DIR=out/dist ./art/tools/dist_linux_bionic.sh \
        -j80 com.android.art.host
  on master-art
Bug: 274756655
Change-Id: I3e7cdc7dc4ed45ffde4ac139b00d1e9e0148744b