fix: macOS link uses system libc++ (not xlings LLVM's)

xlings LLVM's libc++.dylib doesn't pull in libc++abi, causing
undefined exception/RTTI symbols. Solution: don't pass -L<llvm>/lib
to the linker on macOS — let clang++ link against system libc++
(/usr/lib/libc++.dylib) which correctly includes libc++abi.

Compile still uses LLVM 20 headers (ABI-compatible with system libc++).

Author: Sunrisepeak

src/build/flags.cppm

@@ -164,17 +164,16 @@ CompileFlags compute_flags(const BuildPlan& plan) {
 
 #if defined(__APPLE__)
     // macOS linking strategy:
-    // 1. Do NOT pass --sysroot to the linker. The macOS SDK .tbd stubs
-    //    lack libc++abi exports → undefined symbols. Without sysroot,
-    //    clang++ links against /usr/lib system libraries correctly.
-    // 2. Explicitly pass -lc++ because xlings LLVM's clang++.cfg uses
-    //    -nostdinc++ which also suppresses the implicit stdlib linkage.
-    // sysroot is still passed for compilation (finding system headers).
-    std::string stdlib_link = isClang ? " -lc++" : "";
-    f.ld = std::format("{}{}{}{}{}", full_static, static_stdlib, b_flag,
-                       runtime_dirs, stdlib_link);
+    // - No --sysroot: SDK .tbd stubs miss libc++abi exports.
+    // - No -L<llvm>/lib: xlings LLVM's libc++.dylib doesn't pull in
+    //   libc++abi. System /usr/lib/libc++ does (and is ABI-compatible
+    //   with LLVM 20 headers since macOS ships a recent libc++).
+    // - No -rpath for LLVM lib: binary should use system libc++ at runtime.
+    // - Explicit -lc++: clang++.cfg's -nostdinc++ suppresses implicit linkage.
+    // Result: compile with LLVM headers, link with system libc++ + libc++abi.
+    f.ld = std::format("{}{}{} -lc++", full_static, static_stdlib, b_flag);
 #else
-    // Linux: sysroot must be passed to linker (glibc/musl lives there)
+    // Linux: sysroot + runtime dirs needed (glibc/libc++ live in sandbox)
     f.ld = std::format("{}{}{}{}{}", full_static, static_stdlib, sysroot_flag, b_flag,
                        runtime_dirs);
 #endif