Revisit the `rust_test_helpers` setup

ui, incremental test suites all have implicit access to a C helper static library built for the host/target platforms, called rust_test_helpers.

EDIT: two copies, one for host, one for target, see https://github.com/rust-lang/rust/blob/243c5a35e18b2634892fe7091d5ee888a18f77f5/src/bootstrap/src/core/build_steps/test.rs#L1791-L1798.

https://github.com/rust-lang/rust/blob/master/tests/auxiliary/rust_test_helpers.c

https://github.com/search?q=repo%3Arust-lang%2Frust%20rust_test_helpers&type=code

This is kinda strange for a few reasons:

• All tests/{ui,incremental} tests implicitly have the built C static lib in the linker search paths, even if the test doesn't need it
• A test might not need rust_test_helpers, but during bootstrap, it will be unconditionally built for the target anyway (requires a working target C compiler toolchain).
• You can't specify or override what C compiler flags rust_test_helpers is built with. Notably, this includes optimization levels or debuginfo levels, or sanitizers or whatever. Every tests/{ui,incremental} gets access to the same prebuilt copy.

I'm not looking to change this any time soon, since it needs some surveying on what use cases tests using rust_test_helpers are exercising to come up with more robust alternative designs.