Export upd_aggregate

[guibescos]

For one of the latest steps of the migration we're going to restrict the C code to the aggregation logic.
This requires exporting the function upd_aggregate from C to Rust. We're doing similar logic to exporting the c_entrypoint from oracle.c.

A subtle difficulty is that cargo test runs in the system architecture, so we need to compile upd_aggregate to this target as well. That's the goal of for_cargo_test/cpyth_test.c. This will allow using upd_aggregate in the tests.

Of course there's danger that our rust tests pass but our production code fails because they're compiling to different targets but this problem already existed with test_oracle.c which is also getting compiled to the system's target. Integration tests (currently pytest) should pick this up.