test/link: add linker test to verify mangling

This adds a simple linker test to ensure the built library contains
an import entry for each extern function call that was mangled.

test/link.zig

@@ -48,6 +48,11 @@ fn addWasmCases(cases: *tests.StandaloneContext) void {
         .use_emulation = true,
     });
 
+    cases.addBuildFile("test/link/wasm/extern-mangle/build.zig", .{
+        .build_modes = true,
+        .requires_stage2 = true,
+    });
+
     cases.addBuildFile("test/link/wasm/infer-features/build.zig", .{
         .requires_stage2 = true,
     });

test/link/wasm/extern-mangle/a.zig

@@ -0,0 +1 @@
+pub extern "a" fn hello() i32;

test/link/wasm/extern-mangle/b.zig

@@ -0,0 +1 @@
+pub extern "b" fn hello() i32;

test/link/wasm/extern-mangle/build.zig

@@ -0,0 +1,24 @@
+const std = @import("std");
+const Builder = std.build.Builder;
+
+pub fn build(b: *Builder) void {
+    const mode = b.standardReleaseOptions();
+
+    const test_step = b.step("test", "Test");
+    test_step.dependOn(b.getInstallStep());
+
+    const lib = b.addSharedLibrary("lib", "lib.zig", .unversioned);
+    lib.setBuildMode(mode);
+    lib.setTarget(.{ .cpu_arch = .wasm32, .os_tag = .freestanding });
+    lib.install();
+
+    const check_lib = lib.checkObject(.wasm);
+    check_lib.checkStart("Section import");
+    check_lib.checkNext("entries 2"); // a.hello & b.hello
+    check_lib.checkNext("module a");
+    check_lib.checkNext("name hello");
+    check_lib.checkNext("module b");
+    check_lib.checkNext("name hello");
+
+    test_step.dependOn(&check_lib.step);
+}

test/link/wasm/extern-mangle/lib.zig

@@ -0,0 +1,6 @@
+const a = @import("a.zig").hello;
+const b = @import("b.zig").hello;
+export fn foo() void {
+    _ = a();
+    _ = b();
+}