HACKs print linker info

I think we're running an old buggy linker in GitHub Actions.
Let's print the linker info to help debug this.

.github/workflows/apple.yml

@@ -234,6 +234,11 @@ jobs:
 
       - name: Build
         run: |
+          # Our LLVM toolchain doesn't provide a linker. And the /usr/bin/ld on macOS
+          # runners may contain a bug where weak references aren't preserved during LTO.
+          # So force the use of a modern LLVM linker.
+          export FORCE_LINKER=$(brew --prefix llvm)/bin/ld64.lld
+
           if [ "${{ matrix.build.target_triple }}" = "aarch64-apple-darwin" ]; then
             export APPLE_SDK_PATH=/Applications/Xcode_15.2.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX14.2.sdk
           elif [ "${{ matrix.build.target_triple }}" = "x86_64-apple-darwin" ]; then

cpython-unix/build.py

@@ -205,6 +205,21 @@ def add_target_env(env, build_platform, target_triple, build_env):
         extra_host_cflags.extend(["-isysroot", host_sdk_path])
         extra_host_ldflags.extend(["-isysroot", host_sdk_path])
 
+    # Our custom LLVM toolchain doesn't include an lld. Our clang driver
+    # will pick up /usr/bin/ld by default. On some macOS, /usr/bin/ld is
+    # an older version of LLVM and won't preserve weak references when doing
+    # LTO.
+    if "FORCE_LINKER" in os.environ:
+        # We can't use -fuse-ld=/absolute/path. So we deconstruct the full path
+        # and specify the filename in -fuse-ld and put the linker path at the front
+        # of PATH.
+        ld = pathlib.Path(os.environ["FORCE_LINKER"])
+        flag = "-fuse-ld=%s" % ld.name
+
+        env["PATH"] = "%s:%s" % (ld.parent, env["PATH"])
+        extra_host_ldflags.append(flag)
+        extra_target_ldflags.append(flag)
+
     env["EXTRA_HOST_CFLAGS"] = " ".join(extra_host_cflags)
     env["EXTRA_HOST_LDFLAGS"] = " ".join(extra_host_ldflags)
     env["EXTRA_TARGET_CFLAGS"] = " ".join(extra_target_cflags)

cpython-unix/targets.yml

@@ -85,6 +85,7 @@ aarch64-apple-darwin:
     - '-Wno-undef-prefix'
     - '-fvisibility=hidden'
   target_ldflags:
+    - '-v'
     - '-arch'
     - 'arm64'
     - '-mmacosx-version-min=11.0'
@@ -569,6 +570,7 @@ x86_64-apple-darwin:
     - '-Wno-undef-prefix'
     - '-fvisibility=hidden'
   target_ldflags:
+    - '-v'
     - '-arch'
     - 'x86_64'
     - '-mmacosx-version-min=10.9'