Fix bazel builds with unspecified arch

//build/bazel/platforms:android_target is an alias to
android_{arch}, so on bulids like "ndk" that don't
specify an arch, it fails.

It's also questionable if we should be setting the
platform to android in a mixed build in the first
place. What if we were mixed-building a host tool?

This reproduces when running
`OUT_DIR=out ./build/soong/scripts/build-ndk-prebuilts.sh`
with aosp/2263623. That script specifies --soong-only,
which is important because even without bazel, `m nothing`
on ndk-userdebug will fail with errors in makefile code.

Bug: 254701254
Test: Described above
Change-Id: I34cae0287830a19c4865fbd398f3c4e1ee1d2d66