Real cross-compiling tests instead of `#![no_core]` silliness

We have a zillion tests that use the #![no_core] hack, like this one: https://github.com/rust-lang/rust/blob/5e3ede22ef6fd23197629c22b9f466b3734e19f1/tests/ui/asm/bad-template.rs#L10

A given test involves:

• Host A
• Target B
• Some Rust source (compiled for Target B)
• Test tooling (run on Host A)

Most of our test infra assumes that Host A and Target B are the same thing. This greatly simplifies a lot of assumptions. But this hack we have introduced in so many tests essentially exists so that we can be sure the test runs even when cross-compiling from Host A to Target B. It's possible there's some weird finagling of test flags that can enable this without the hack, but that would be another hack itself. Cross-compiled tests should be relatively easy, like just writing something like:

//@ cross-compile-targets: arch_1-unknown-os_3-env_5 arch_2-unknown-os_4-env_6 arch_1-unknown-os_4-env_5 arch_2-unknown-os_3-env_7

Start with tests/ui/asm/bad-template.rs and inspect the surrounding test infrastructure assumptions about Host A and Target B. Trace how a cross-compile-targets directive could select targets for a test, and define done as cross-compiled tests running without the #![no_core] workaround.