macos: add tbd-v3 zippered support

Support linking against tbd-v3 SDKs such as those bundled with
Xcode 10.3 → 11.3.1 .

- Map target os=`ios` and abi=`macabi` to macho.PLATFORM.MACCATALYST.
  This allows for matches against tbdv4 targets, eg. `x86_64-maccatalyst`.

- When parsing old tbdv3 files with `zippered` platform, we append
  [`ARCH-macos`, `ARCH-maccatalyst`] to list of "tbd" targets. This
  enables linking for standard targets like `ARCH-macos-none` and
  maccatalyst targets `ARCH-ios-macabi`.

- Update mappings for macho platform and zig target macabi. While this
  is not full maccatalyst support, a basic exe can be built as follows:

```
zig libc > libc.txt
zig build-exe z0.zig --libc libc.txt -target x86_64-ios-macabi
```

closes #19110

src/link/MachO.zig

@@ -4376,9 +4376,11 @@ pub const Platform = struct {
                         .IOS, .IOSSIMULATOR => .ios,
                         .TVOS, .TVOSSIMULATOR => .tvos,
                         .WATCHOS, .WATCHOSSIMULATOR => .watchos,
+                        .MACCATALYST => .ios,
                         else => @panic("TODO"),
                     },
                     .abi = switch (cmd.platform) {
+                        .MACCATALYST => .macabi,
                         .IOSSIMULATOR,
                         .TVOSSIMULATOR,
                         .WATCHOSSIMULATOR,
@@ -4425,7 +4427,11 @@ pub const Platform = struct {
     pub fn toApplePlatform(plat: Platform) macho.PLATFORM {
         return switch (plat.os_tag) {
             .macos => .MACOS,
-            .ios => if (plat.abi == .simulator) .IOSSIMULATOR else .IOS,
+            .ios => switch (plat.abi) {
+                .simulator => .IOSSIMULATOR,
+                .macabi => .MACCATALYST,
+                else => .IOS,
+            },
             .tvos => if (plat.abi == .simulator) .TVOSSIMULATOR else .TVOS,
             .watchos => if (plat.abi == .simulator) .WATCHOSSIMULATOR else .WATCHOS,
             else => unreachable,

src/link/MachO/Dylib.zig

@@ -751,8 +751,14 @@ pub const TargetMatcher = struct {
             .v3 => |v3| blk: {
                 var targets = std.ArrayList([]const u8).init(arena.allocator());
                 for (v3.archs) |arch| {
-                    const target = try std.fmt.allocPrint(arena.allocator(), "{s}-{s}", .{ arch, v3.platform });
-                    try targets.append(target);
+                    if (mem.eql(u8, v3.platform, "zippered")) {
+                        // From Xcode 10.3 → 11.3.1, macos SDK .tbd files specify platform as 'zippered'
+                        // which should map to [ '<arch>-macos', '<arch>-maccatalyst' ]
+                        try targets.append(try std.fmt.allocPrint(arena.allocator(), "{s}-macos", .{arch}));
+                        try targets.append(try std.fmt.allocPrint(arena.allocator(), "{s}-maccatalyst", .{arch}));
+                    } else {
+                        try targets.append(try std.fmt.allocPrint(arena.allocator(), "{s}-{s}", .{ arch, v3.platform }));
+                    }
                 }
                 break :blk targets.items;
             },