[stdlib] Fix incorrect Float16 extension

This extension (introduced in #35264) was placed in a file location where it wasn’t correctly guarded against mentioning Float16 on macOS/x86_64, so the generated .swiftinterface file included a reference to an unavailable declaration. (The dummy stand-in Float16 type that we currently use on Intel macOS.)

Moving the declaration out of the “AnyHashable” section and into a file region that’s more suitable for it (i.e., enclosed in `#if !((os(macOS) || targetEnvironment(macCatalyst)) && arch(x86_64))`) resolves the issue.

rdar://76025365

Author: lorentey

stdlib/public/core/FloatingPointTypes.swift.gyb

@@ -1019,6 +1019,9 @@ extension ${Self} {
   }
 }
 
+${Availability(bits)}
+extension ${Self}: Sendable { }
+
 //===----------------------------------------------------------------------===//
 // Explicit conversions between types.
 //===----------------------------------------------------------------------===//
@@ -1343,9 +1346,6 @@ internal struct _${Self}AnyHashableBox: _AnyHashableBox {
 }
 % end
 
-${Availability(bits)}
-extension ${Self} : Sendable { }
-
 //===----------------------------------------------------------------------===//
 // Deprecated operators
 //===----------------------------------------------------------------------===//