all: add the Boehm-Demers-Weiser GC on Linux

This adds support for the well-known Boehm GC. It's significantly faster
than our own naive GC and could be used as an alternative on bigger
systems.

In the future, this GC might also be supported on WebAssembly with some
extra work. Right now it's Linux only (though Windows/MacOS shouldn't be
too difficult to add).

.github/workflows/nix.yml

@@ -22,9 +22,9 @@ jobs:
         run: sudo apt-get remove llvm-18
       - name: Checkout
         uses: actions/checkout@v4
-      - name: Pull musl
+      - name: Pull musl, bdwgc
         run: |
-            git submodule update --init lib/musl
+            git submodule update --init lib/musl lib/bdwgc
       - name: Restore LLVM source cache
         uses: actions/cache/restore@v4
         id: cache-llvm-source