Abi compatibility test cleanup

[folkertdev]

r? @jieyouxu

• moves NonNull (and some friends) into minicore.rs so it can be re-used.
• tests the abi compatibility of a couple more targets. It's useful to have a comprehensive overview of all ABIs that rust has some amount of support for, e.g. testing for c-variadic or guaranteed tail calls (cc riscv64: rustc-LLVM ERROR: failed to perform tail call elimination on a call site marked musttail #148748)

[rustbot]

This PR modifies tests/auxiliary/minicore.rs.

cc @jieyouxu