macho: use TOOL=0x5 to mean ZIG as the build tool

lib/std/macho.zig

@@ -143,6 +143,8 @@ pub const TOOL = enum(u32) {
     CLANG = 0x1,
     SWIFT = 0x2,
     LD = 0x3,
+    LLD = 0x4, // LLVM's stock LLD linker
+    ZIG = 0x5, // Unofficially Zig
     _,
 };
 

src/link/MachO/load_commands.zig

@@ -294,7 +294,7 @@ pub fn writeBuildVersionLC(options: *const link.Options, lc_writer: anytype) !vo
         .ntools = 1,
     });
     try lc_writer.writeAll(mem.asBytes(&macho.build_tool_version{
-        .tool = .LD,
+        .tool = .ZIG,
         .version = 0x0,
     }));
 }