link-tests: move macho tests to subfolder

Handle `-e` option in MachO linker allowing the user to set a custom
entrypoint address.

test/link/macho/objcpp/Foo.h

@@ -0,0 +1,7 @@
+#import <Foundation/Foundation.h>
+
+@interface Foo : NSObject
+
+- (NSString *)name;
+
+@end

test/link/macho/objcpp/Foo.mm

@@ -0,0 +1,11 @@
+#import "Foo.h"
+
+@implementation Foo
+
+- (NSString *)name
+{
+      NSString *str = [[NSString alloc] initWithFormat:@"Zig"];
+      return str;
+}
+
+@end

test/link/macho/objcpp/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 the program");
+
+    const exe = b.addExecutable("test", null);
+    b.default_step.dependOn(&exe.step);
+    exe.addIncludePath(".");
+    exe.addCSourceFile("Foo.mm", &[0][]const u8{});
+    exe.addCSourceFile("test.mm", &[0][]const u8{});
+    exe.setBuildMode(mode);
+    exe.linkLibCpp();
+    // TODO when we figure out how to ship framework stubs for cross-compilation,
+    // populate paths to the sysroot here.
+    exe.linkFramework("Foundation");
+
+    const run_cmd = exe.run();
+    run_cmd.expectStdOutEqual("Hello from C++ and Zig");
+
+    test_step.dependOn(&run_cmd.step);
+}

test/link/macho/objcpp/test.mm

@@ -0,0 +1,14 @@
+#import "Foo.h"
+#import <assert.h>
+#include <iostream>
+
+int main(int argc, char *argv[])
+{
+  @autoreleasepool {
+      Foo *foo = [[Foo alloc] init];
+      NSString *result = [foo name];
+      std::cout << "Hello from C++ and " << [result UTF8String];
+      assert([result isEqualToString:@"Zig"]);
+      return 0;
+  }
+}