Update docs to highlight the ad-hoc process for riscv64.

Also allow filtering with --local-dist-riscv64. It's useful to filter
there as well, to avoid having to build more than necessary, and the
filters should work equally well after `install_entries` has returned.

Test: prebuilts/runtime/mainline/update.py \
        --local-dist-riscv64 out/dist --skip-apex
  where APEX'es haven't been built in out/dist
Bug: 359159472
Change-Id: Ia472ff449f7a93bef24163d0cef684d0d9b8eceb
2 files changed