builtin-libc++ not working on macos

[Icantjuddle]

I've configure a barebones llvm toolchain via:

llvm = use_extension("@toolchains_llvm//toolchain/extensions:llvm.bzl", "llvm")
llvm.toolchain(
     cxx_standard = {"": "c++23"},
    stdlib = {"": "builtin-libc++"},
     llvm_version = "21.1.6",
 )
 use_repo(llvm, "llvm_toolchain")
 register_toolchains("@llvm_toolchain//:all")

However when linking my application it appears to be using the system libc++ not the one from the toolchain as it is unable to find symbols used by the version of the standard library, in particular: undefined symbol: std::__1::__hash_memory.

I verified with:
nm -gU /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/lib/libc++.tbd 2>/dev/null | grep __hash_memory and nm -gU bazel-whenufree/external/toolchains_llvm++llvm+llvm_toolchain_llvm/lib/libc++.a 2>/dev/null | grep __hash_memory that the symbol is only present in the builtin version of libc++.

Looking in the link params file I see this snippet which looks off to me, I would expect the order in which search paths are added to have the builtin library ahead of the system library so it is preferred:

#... more
-L/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/lib
-lc++
-lc++abi
-Bdynamic
-Lexternal/toolchains_llvm++llvm+llvm_toolchain_llvm/lib
# ... more

Moving the -Lexternal/toolchains_llvm++llvm+llvm_toolchain_llvm/lib above the first line allows the program to link and run.