Add support for cross-compilation

This adds proper support for cross-compiling Inko programs from one
target to another.

The Rust runtime is compiled ahead of time as part of the release
process. These runtimes are managed using the `inko runtime` command.
For musl we also include libunwind.a as this is needed to resolve linker
symbol errors. The library is taken from the rustup musl toolchain to
ensure it's the same version as Rust expects. This also saves us having
to compile it in the first place.

By default the compiler tries to detect what compiler/linker driver to
use based on the target you're compiling for. For example, when
targeting musl, the linker tries to use musl-gcc or musl-clang if it's
installed. A custom linker can be specified using the --linker option.
We also support linking using Zig, which makes it _much_ easier to
target macOS and potentially other targets, provided you have Zig
installed of course.

This fixes #524.

Changelog: added

.github/workflows/release.yml

@@ -66,6 +66,26 @@ jobs:
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
 
+  runtimes:
+    runs-on: ubuntu-latest
+    needs:
+      - lints
+      - linux
+      - mac
+      - freebsd
+    env:
+      AWS_REGION: eu-west-1
+      AWS_ACCESS_KEY_ID: ${{ secrets.AWS_ACCESS_KEY_ID }}
+      AWS_SECRET_ACCESS_KEY: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
+    steps:
+      - uses: actions/checkout@v4
+      - name: Installing dependencies
+        run: sudo ./scripts/deps.sh ubuntu:latest
+      - name: Installing Rust
+        run: ./scripts/rust.sh 1.70
+      - name: Generating runtimes
+        run: make runtimes
+
   docs:
     runs-on: ubuntu-latest
     needs: