[embedded] Use #if instead of unavailability for _undefined

Author: kubamracek

stdlib/public/core/AssertCommon.swift

@@ -252,8 +252,9 @@ func _unimplementedInitializer(className: StaticString,
   Builtin.int_trap()
 }
 
+#if !$Embedded
+
 /// Used to evaluate editor placeholders.
-@_unavailableInEmbedded
 public // COMPILER_INTRINSIC
 func _undefined<T>(
   _ message: @autoclosure () -> String = String(),
@@ -262,6 +263,8 @@ func _undefined<T>(
   _assertionFailure("Fatal error", message(), file: file, line: line, flags: 0)
 }
 
+#else
+
 /// Used to evaluate editor placeholders.
 public // COMPILER_INTRINSIC
 func _undefined<T>(
@@ -271,6 +274,8 @@ func _undefined<T>(
   _assertionFailure("Fatal error", message(), file: file, line: line, flags: 0)
 }
 
+#endif
+
 /// Called when falling off the end of a switch and the type can be represented
 /// as a raw value.
 ///