Replace commented-out deprecated attribute with doc comment.

This attribute had been commented out since it was first
committed a decade ago. No-one's going to chase down arm32-only
trivia like that before arm32 and x86 are removed as obsolete...

Change-Id: I8e29d16657b3f4ce379e65732d747f969ea31dae
diff --git a/libc/include/unistd.h b/libc/include/unistd.h
index c69db61..2732214 100644
--- a/libc/include/unistd.h
+++ b/libc/include/unistd.h
@@ -350,8 +350,11 @@
 int daemon(int __no_chdir, int __no_close);
 
 #if defined(__arm__)
+/**
+ * New code should use __builtin___clear_cache() instead, which works on
+ * all architectures.
+ */
 int cacheflush(long __addr, long __nbytes, long __cache);
-    /* __attribute__((deprecated("use __builtin___clear_cache instead"))); */
 #endif
 
 pid_t tcgetpgrp(int __fd);