Allow compile_multilib to be specified on module exports

This is needed to allow the art-host-module-exports to restrict itself
to just managing the linux 64 bit version of the host tools as that is
the only variant that is currently supported by all host tools. This
greatly simplifies that process and allows us to make progress on the
unbundling.

Bug: 142935992
Test: m nothing
Change-Id: I62d016d97c2df73e5feecf912638f477fedd97c9