Fwiw, you can use the underlying LLVM simd builtins on Linux by passing the -parse-stdlib-flag to swift.
It's quite unwieldy to use of course, but could be used to build a nicer-to-use simd library usable on Linux (although that might not work with SwiftPM right now: Compiler flags in Package.swift).
I'm not saying wether or not Swift should come with simd support built-in, but you can play around with this or use it if you really need it.