tests: check wasm compiler_builtins object architecture

Add a run-make test that checks the `.o` members in the prebuilt
`libcompiler_builtins` archive for `wasm32-wasip1` are wasm objects.

This guards against building wasm compiler-rt fallbacks with the host C
toolchain.

Author: Vastargazing

tests/run-make/wasm-compiler-builtins-object-arch/rmake.rs

@@ -0,0 +1,49 @@
+//! Regression test for <https://github.com/rust-lang/rust/issues/132802>.
+//!
+//! The prebuilt `libcompiler_builtins` rlib bundled in the wasm sysroot must
+//! contain wasm object files — never host ELF/Mach-O/COFF. Bootstrap could
+//! previously pick the host C toolchain for compiler-rt fallbacks on wasm
+//! targets and silently embed host objects into the wasm sysroot
+//! (fixed in rust-lang/rust#137457).
+
+//@ only-wasm32
+
+use run_make_support::object::read::archive::ArchiveFile;
+use run_make_support::{has_extension, has_prefix, rfs, rustc, shallow_find_files};
+
+fn main() {
+    let libdir = rustc().print("target-libdir").run().stdout_utf8();
+    let libdir = libdir.trim();
+
+    let rlibs = shallow_find_files(libdir, |path| {
+        has_prefix(path, "libcompiler_builtins") && has_extension(path, "rlib")
+    });
+    assert!(!rlibs.is_empty(), "no libcompiler_builtins rlib found in {libdir}");
+
+    let data = rfs::read(&rlibs[0]);
+    let archive = ArchiveFile::parse(&*data).unwrap();
+
+    let mut checked = 0usize;
+    for member in archive.members() {
+        let member = member.unwrap();
+        let name = std::str::from_utf8(member.name()).unwrap_or("<invalid-utf8>");
+        if !name.ends_with(".o") {
+            continue;
+        }
+        let obj_data = member.data(&*data).unwrap();
+        assert!(
+            obj_data.starts_with(b"\0asm"),
+            "object `{name}` in compiler_builtins rlib is not a wasm object \
+             (first bytes: {:02x?}) — see rust-lang/rust#132802",
+            &obj_data[..4.min(obj_data.len())]
+        );
+        checked += 1;
+    }
+
+    assert!(
+        checked > 0,
+        "no .o members found in compiler_builtins rlib at {} — \
+         archive should always contain object files",
+        rlibs[0].display(),
+    );
+}