BridgeJS: Gate @_extern/@expose usage behind `arch(wasm32)` #1529

Job	Run time
Build for native target (macos-15, Xcode_16)	31s
Build and Test (ubuntu-22.04, https://download.swift.org/swift-6.1-release/ubuntu2204/swift-6.1-R...	4m 33s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	6m 6s
format	32s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	5m 51s
Build and Test (ubuntu-22.04, https://download.swift.org/swift-6.0.2-release/ubuntu2204/swift-6.0...	4m 13s
	21m 46s

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