Cross-compile linuxArm64 tests on x86_64 (was: failing on arm64 host)

[IRus]

Problem

The `Linux arm64 tests` matrix entries (in `ci.yml`) and `Linux arm64 Native` (in `pinned-conformance.yml`) fail at Gradle configuration time:

```

A problem occurred configuring project ':benchmarks'.
Caused by: org.jetbrains.kotlin.konan.target.TargetSupportException:
Unknown host target: linux aarch64
```

Root cause: Kotlin/Native (Konan) does not publish a linux-aarch64 host toolchain in any version. `HostManager` cannot determine the host on `ubuntu-24.04-arm`, so Gradle cannot configure `:benchmarks` (or any project declaring a K/N target). This blocks every gradle command on that runner, not just the arm64 tests.

Why "just run linuxArm64Test" is not possible

Even if the Konan host problem were worked around (QEMU on x86_64, etc.), the `linuxArm64Test` task does not exist on any host. `KotlinMultiplatformTargetPresetSetupAction` only registers a host test-run task for `{LINUX_X64, MACOS_X64, MACOS_ARM64, MINGW_X64}`. `LINUX_ARM64` instantiates a plain `KotlinNativeTarget` with no test-run, so `tasks.register("linuxArm64Test", ...)` is never called. The CI invocation `:core:linuxArm64Test` would fail with "task not found" regardless of host. (kotlinx-benchmark has the same limitation for `linuxArm64Benchmark`.)

Fix

Move the matrix entries onto `ubuntu-24.04` (x86_64, where Konan's host is supported) and cross-compile the linuxArm64 test compilation instead of running it:

```
:core:compileTestKotlinLinuxArm64 (was :core:linuxArm64Test)
:snapshot:compileTestKotlinLinuxArm64
:wasi:compileTestKotlinLinuxArm64
:bindgen-api:compileTestKotlinLinuxArm64
:bindgen-runtime:compileTestKotlinLinuxArm64
:tck:compileTestKotlinLinuxArm64
```

This validates that every module's `commonTest`/`linuxArm64` source compiles against the aarch64 Kotlin/Native toolchain on every PR — catching target-specific signature/API errors — without depending on a host that does not exist.

What this does NOT cover

• Runtime execution of aarch64 test binaries is out of scope. KGP provides no DSL to run them cross-host; doing so would require a custom `Exec` task wrapping `qemu-aarch64-static` + JUnit-XML parsing — a follow-up if runtime arm64 coverage is wanted.
• linuxArm64 benchmarks remain without CI coverage (kotlinx-benchmark does not register them on a non-matching host; same class of limitation).

Both are tracked upstream as KT-36871 (linux aarch64 host support).

Verification

```
./gradlew :core:compileTestKotlinLinuxArm64 :tck:compileTestKotlinLinuxArm64
-> BUILD SUCCESSFUL
```

All six `compileTestKotlinLinuxArm64` tasks resolve and cross-compile against the linuxArm64 klib toolchain. YAML for both workflows validated.