Rust cdylib/statliclib support for vendor snapshot.

Adds support for platform vendor_available Rust FFI libraries and
binaries to be included in the vendor snapshot.

Because rlib and dylibs are not yet in snapshots, libstd cannot be
included in a vendor snapshot. As a result, vendor-specific Rust code
can't be guaranteed to work with the platform-provided vendor_available
modules built with a newer toolchain. For now, a check is added
indicating vendor-specific Rust code is unsupported.

This changes the linkage for vendor variants of these modules to default
to rlib linkage since dylibs cannot be included in the snapshot yet.

Bug: 184042776
Test: m nothing # new Soong tests pass
Change-Id: I502eaa4bb962eb87ff868fcf49b435f0d2f982e6