Currently proc_macros don't pull in the correct arch dependencies when
the host arch differs from the target arch.
This fixes how proc_macro dependencies are handled by defining them as
always being host-only and including them as dependencies for
device-modules by using AddFarVariationDependencies.
Bug: 141491501
Test: Example device rust module builds with proc_macro dependency.
Change-Id: Ic037dc406ce90526f8b68c92fffc0d93a498a4ff
Adds support to Soong for building rust modules. This currently only
supports x86_64 device and x86 linux host targets. The functionality
is sufficient to build crosvm.
Bug: 136189233
Test: Test module builds.
Test: crosvm builds.
Change-Id: I6ea04615834a6d673578ab10ea1a2eb04259fe09
