builder: add support for cross compiling to Darwin

This means that it will be possible to generate a Darwin binary on any
platform (Windows, Linux, and MacOS of course), including CGo. Of
course, the resulting binaries can only run on MacOS itself.

The binary links against libSystem.dylib, which is a shared library. The
macos-minimal-sdk repository contains open source header files and
generated symbol stubs so we can generate a stub libSystem.dylib without
copying any closed source code.

.github/workflows/linux.yml

@@ -51,7 +51,7 @@ jobs:
         uses: actions/cache@v2
         id: cache-llvm-build
         with:
-          key: llvm-build-13-linux-v1
+          key: llvm-build-13-linux-v2
           path: llvm-build
       - name: Build LLVM
         if: steps.cache-llvm-build.outputs.cache-hit != 'true'
@@ -192,7 +192,7 @@ jobs:
         uses: actions/cache@v2
         id: cache-llvm-build
         with:
-          key: llvm-build-13-linux-asserts-v1
+          key: llvm-build-13-linux-asserts-v2
           path: llvm-build
       - name: Build LLVM
         if: steps.cache-llvm-build.outputs.cache-hit != 'true'