Everywhere: Replace uses of __serenity__ with AK_OS_SERENITY

Now that we have OS macros for essentially every supported OS, let's try
to use them everywhere.

Userland/Libraries/LibCore/File.cpp

@@ -4,9 +4,6 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
-#ifdef __serenity__
-#    include <serenity.h>
-#endif
 #include <AK/LexicalPath.h>
 #include <AK/ScopeGuard.h>
 #include <LibCore/DirIterator.h>
@@ -20,6 +17,10 @@
 #include <unistd.h>
 #include <utime.h>
 
+#ifdef AK_OS_SERENITY
+#    include <serenity.h>
+#endif
+
 // On Linux distros that use glibc `basename` is defined as a macro that expands to `__xpg_basename`, so we undefine it
 #if defined(__linux__) && defined(basename)
 #    undef basename
@@ -241,7 +242,7 @@ String File::absolute_path(String const& path)
     return LexicalPath::canonicalized_path(full_path.string());
 }
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
 
 ErrorOr<String> File::read_link(String const& link_path)
 {