macOS

Introduce MODULAR_GC_FN #640

Job	Run time
make (check, macos-14)	5m 53s
make (check, macos-14, --with-gcc=gcc-14)	7m 51s
make (check, macos-14, --with-gmp)	6m 45s
make (check, macos-14, --with-jemalloc --with-opt-dir=$(brew --prefix jemalloc))	6m 12s
make (check, macos-15)	5m 45s
make (test-all, --repeat-count=2, macos-14)	8m 4s
make (check, macos-13)	14m 6s
make (test-bundler-parallel, macos-14)	17m 11s
make (test-bundled-gems, macos-14)	10m 0s
macOS result	3s
	1h 21m 50s

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