update unavailability message

Author: Dante-Broggi

stdlib/public/core/FloatingPointTypes.swift.gyb

@@ -1729,7 +1729,7 @@ extension ${Self} {
 
 ${SelfDocComment}
 @_fixed_layout
-@available(*, unavailable, message: "available only #if !os(Windows) && (arch(i386) || arch(x86_64))")
+@available(*, unavailable, message: "Float80 is only available on non-Windows x86 targets.")
 public struct ${Self} {
   /// Creates a value initialized to zero.
   @_inlineable // FIXME(sil-serialize-all)