fix(macOS): respect MTU on macOS in udp-large-recv test-case

[marxin]

No description provided.

[Copilot]

Pull request overview

This PR fixes the udp-large-recv WASIX socket test on macOS by ensuring the UDP payload size respects macOS loopback MTU limitations, while preserving the larger payload on Linux where loopback MTU is larger.

Changes:

• Introduces a build.sh for udp-large-recv that compiles main.c with an OS-dependent PAYLOAD_SIZE (smaller on macOS).
• Updates the WASIX wasm_tests harness to export WASMER_HOST_OS for build steps, enabling build scripts to tune compilation based on host OS.
• Moves the test’s ExpectedStdout directive from main.c into the new build.sh (matching the harness’ ## directive format for .sh sources).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
lib/wasix/tests/wasm_tests/socket/udp-large-recv/main.c	Removes hardcoded PAYLOAD_SIZE and relies on build-time -D from the build script.
lib/wasix/tests/wasm_tests/socket/udp-large-recv/build.sh	New build script selects payload size by host OS and compiles main.c accordingly; carries ExpectedStdout.
lib/wasix/tests/wasm_tests/mod.rs	Documents and implements WASMER_HOST_OS injection into the build environment.

[marxin]

Noticed #6785.

[marxin]

Superseded by #6789.