Add compile_multilib properties to all native prebuilts.

There was a special case in cc_prebuilt_binary only, which resulted in
prebuilt libraries getting both 32 and 64 bit variants even when their
sources only had one of them, and the other variant would be defunct
since it wouldn't get any prebuilt artefact.

This moves the handling of compile_multilib completely to the common
update code, so that SDK members don't need to deal with it. It doesn't
take SDK member defaults into account, which means a bit more
boilerplate in the snapshots, but it's simpler and less error prone
(different SDK member types have different defaults).

Bug: 151303681
Test: m nothing
Test: build/soong/scripts/build-aml-prebuilts.sh art-module-host-exports
  Check that the generated Android.bp passes Soong.
Change-Id: Ib73444c6788ee1c78480bdb103aa2b8ae8f2c63c