Use Float16 only for arm64

Sources/JSONAST/JSON.Node.swift

@@ -224,10 +224,12 @@ extension JSON.Node {
     /// -   Returns:
     ///     The closest value of ``Float16`` to the payload of this variant if it matches
     ///     ``number(_:) [case]``, `nil` otherwise.
+    #if arch(arm64)
     @available(macOS 11.0, iOS 14.0, tvOS 14.0, watchOS 7.0, *)
     public func `as`(_: Float16.Type) -> Float16? {
         self.as(JSON.Number.self)?.as(Float16.self)
     }
+    #endif
 
     /// Attempts to load an instance of ``Number`` from this variant.
     ///

Sources/JSONAST/JSON.Number.swift

@@ -156,10 +156,12 @@ extension JSON.Number {
 
     /// Converts this numeric literal to a ``Float16`` value, or its closest
     /// floating-point representation.
+    #if arch(arm64)
     @available(macOS 11.0, iOS 14.0, tvOS 14.0, watchOS 7.0, *)
     public func `as`(_: Float16.Type) -> Float16? {
         self.parsed(as: Float16.self)
     }
+    #endif
 }
 extension JSON.Number {
     /// We want floating point types to roundtrip losslessly, the only way to guarantee that is

Sources/JSONDecoding/Conformances/Float16 (ext).swift

@@ -1,6 +1,8 @@
+#if arch(arm64)
 @available(macOS 11.0, iOS 14.0, tvOS 14.0, watchOS 7.0, *)
 extension Float16: JSONDecodable {
     @inlinable public init(json: borrowing JSON.Node) throws {
         self = try json.cast { $0.as(Self.self) }
     }
 }
+#endif

Sources/JSONEncoding/Conformances/Float16 (ext).swift

@@ -1,2 +1,4 @@
+#if arch(arm64)
 @available(macOS 11.0, iOS 14.0, tvOS 14.0, watchOS 7.0, *)
 extension Float16: JSONEncodable {}
+#endif