Cross-build target for macos compilation

[TheRayquaza]

Hey runwasi folks 👋

I have this tiny PR for enabling compilation on macos using cross

Usage

# Build all runtimes for x86_64-unknown-linux-musl
make cross-build

# Build a single shim with release mode
OPT_PROFILE="release" make cross-build-wasmtime

# Override target arch
make cross-build-wasmtime CROSS_BUILD_TARGET=aarch64-unknown-linux-musl

Test plan

• make cross-build builds all runtimes in RUNTIMES
• make build unaffected (no regression)

[jprendes]

Hi!

I dont see how this change enables aarch64 support. Also aarch64 should already be supported.

Is there a missing commit in the PR maybe?

[TheRayquaza]

Hey @jprendes, you're right that this isn't exactly about adding aarch64 target support. Let me rephrase:

Problem: make build fails entirely on macOS because procfs v0.17.0 only supports Linux/Android. There's no workaround, it uses linux syscalls.

Solution: cross wraps cargo in docker, so the actual compilation happens on linux. The DOCKER_DEFAULT_PLATFORM solves the macos default pulling arm64 images. We need amd64 image because the cross-rs toolchain images are amd64 (without the override, the arm64 image is selected and the build fails).

The new cross-build / cross-build-* targets are just a replacement for running cross instead of cargo, which work for macos (should also work for other platforms I think 🤔)