Add x86_64 cross-compile makefile for ARM64 runners

The macos-14 runner is ARM64, so we need cross-compile flags
to build x86_64 binaries for the universal binary.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: gijzelaerr

.github/build_scripts/x86_64_osx.mk

@@ -0,0 +1,10 @@
+TargetCPU  :=x86_64
+OS         :=osx
+CXXFLAGS   := -O3 -fPIC -pedantic -target x86_64-apple-darwin
+
+# Standard part
+
+include common.mk
+
+# Override the variable to add a target flag
+SharedObjectLinkerName :=g++ -shared -fPIC --target=x86_64-apple-darwin

.github/workflows/osx-build-test-amd64.yml

@@ -20,6 +20,7 @@ jobs:
       - name: Prepare files
         run: |
           cp .github/build_scripts/arm64_osx.mk snap7-full-1.4.2/build/osx/
+          cp .github/build_scripts/x86_64_osx.mk snap7-full-1.4.2/build/osx/
           pushd snap7-full-1.4.2/build/osx/
           make -f x86_64_osx.mk all
           make -f arm64_osx.mk all