Use proper feature check style for this

Author: DougGregor

stdlib/public/Observation/Sources/Observation/Macros.swift

@@ -9,7 +9,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-#if hasFeature(Macros) && hasAttribute(attached)
+#if $Macros && hasAttribute(attached)
 
 @available(SwiftStdlib 5.9, *)
 @attached(member, names: named(_registrar), named(transactions), named(changes), named(_Storage), named(_storage))

stdlib/public/core/Macros.swift

@@ -10,7 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-#if hasFeature(Macros) && hasAttribute(attached)
+#if $Macros && hasAttribute(attached)
 
 /// Specifies the module and type name for an externally-defined macro, which
 /// must conform to the appropriate set of `Macro` protocols.