Document support for Apple Silicon Macs

PiperOrigin-RevId: 347401695
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 ## Supported Architectures
 
-- ARM64 on Android, Linux, and iOS (including WatchOS and tvOS)
+- ARM64 on Android, Linux, macOS, and iOS (including WatchOS and tvOS)
 - ARMv7 (with NEON) on Android, Linux, and iOS (including WatchOS)
 - x86 and x86-64 (up to AVX512) on Windows, Linux, macOS, Android, and iOS simulator
 - WebAssembly MVP