Support watchOS arm64_32 build variants for String.UTF8View

[agnosticdev]

SwiftNIO QUIC has a macOS Tests Swift 6.3 build variant that is failing due to arm64_32 builds for the watchOS toolchain tripping up on Span in the String.UTF8View type.

/Users/runner/Library/Developer/Xcode/DerivedData/swift-nio-quic-aizjhmmkhpwybjavsacfgpunzsnd/SourcePackages/checkouts/swift-network-evolution/Sources/SwiftNetwork/Utilities/Serializer.swift:545:36: error: 'span' is unavailable in watchOS
        let utf8Bytes = value.utf8.span.bytes
                                   ^~~~
Swift.String.UTF8View.span:4:16: note: 'span' has been explicitly marked unavailable here
    public var span: Span<UTF8.CodeUnit> { get }  }
               ^
/Users/runner/Library/Developer/Xcode/DerivedData/swift-nio-quic-aizjhmmkhpwybjavsacfgpunzsnd/SourcePackages/checkouts/swift-network-evolution/Sources/SwiftNetwork/Utilities/Serializer.swift:760:36: error: 'span' is unavailable in watchOS
        let utf8Bytes = value.utf8.span.bytes.extracting(first: byteCount)
                                   ^~~~
Swift.String.UTF8View.span:4:16: note: 'span' has been explicitly marked unavailable here
    public var span: Span<UTF8.CodeUnit> { get }  }

This is in-fact not supported by the compiler, see here:
https://github.com/swiftlang/swift/blob/51b0b4aa41f28eae7d96af6f98c1fbd2b4b63958/stdlib/public/core/StringUTF8View.swift#L386

This change adds a compiler directive to check for arm64_32 variants of watchOS and provide compatibility support there. All other 64 bit variants and other platforms use Span in a normal fashion.