Swift: add flags and instructions for building on macOS ARM

.bazelrc

@@ -11,7 +11,8 @@ common --override_module=semmle_code=%workspace%/misc/bazel/semmle_code_stub
 build --repo_env=CC=clang --repo_env=CXX=clang++
 
 build:linux --cxxopt=-std=c++20
-build:macos --cxxopt=-std=c++20 --cpu=darwin_x86_64
+# we currently cannot built the swift extractor for ARM
+build:macos --cxxopt=-std=c++20 --copt=-arch --copt=x86_64 --linkopt=-arch --linkopt=x86_64
 build:windows --cxxopt=/std:c++20 --cxxopt=/Zc:preprocessor
 
 # this requires developer mode, but is required to have pack installer functioning

swift/README.md

@@ -16,7 +16,14 @@ brew install bazelisk
 then from the `ql` directory run
 
 ```bash
-bazel run //swift:create-extractor-pack    # --cpu=darwin_x86_64 # Uncomment on Arm-based Macs
+bazel run //swift:create-extractor-pack
+```
+
+If you are running on macOS and you encounter errors mentioning `XXX is unavailable: introduced in macOS YY.ZZ`,
+you will need to run this from the root of your `codeql` checkout:
+
+```bash
+echo common --macos_sdk_version=$(sw_vers --productVersion) >> local.bazelrc
 ```
 
 which will install `swift/extractor-pack`.