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

target/board/linux_bionic/BoardConfig.mk

@@ -17,7 +17,11 @@
 # (device) target architectures are irrelevant. However, the build system isn't
 # prepared to handle no target architectures at all, so pick something
 # arbitrarily.
-TARGET_ARCH_SUITE := ndk
+TARGET_ARCH := arm
+TARGET_ARCH_VARIANT := armv7-a-neon
+TARGET_CPU_VARIANT := generic
+TARGET_CPU_ABI := armeabi-v7a
+TARGET_CPU_ABI2 := armeabi
 
 HOST_CROSS_OS := linux_bionic
 HOST_CROSS_ARCH := x86_64