HACK: use arm64 apex prebuilts for riscv64

There are no riscv64 apex prebuilts, which breaks the build when
provenance_metadata singelton tries to run a tool on all the input
apexes that don't exist.  Hack it to provide the arm64 apex for now
if no riscv64 apex is specified, which will allow the build to pass.

Test: lunch aosp_riscv64-userdebug && m droid
Change-Id: Ic39936539803615ea4b7f817daf7b59ab7c40e7e