clang doesn't propagate attributes like "asm labels" and "visibility(hidden)"

to builtins like mem{set,cpy,move} and __stack_smash_handler.  So, when
building with clang, instead mark those as protected visibility to get rid
of the PLT relocations.  We can't take the address of them then, but that's
ok: it's a build-time error not a run-time error.
ok kettenis@

src/lib/libc/string/memcpy.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: memcpy.c,v 1.3 2017/08/14 17:10:02 guenther Exp $ */
+/*	$OpenBSD: memcpy.c,v 1.4 2017/11/29 05:13:57 guenther Exp $ */
 /*-
  * Copyright (c) 1990 The Regents of the University of California.
  * All rights reserved.
@@ -106,4 +106,4 @@ memcpy(void *dst0, const void *src0, size_t length)
 done:
 	return (dst0);
 }
-DEF_STRONG(memcpy);
+DEF_BUILTIN(memcpy);

src/lib/libc/string/memmove.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: memmove.c,v 1.2 2015/08/31 02:53:57 guenther Exp $ */
+/*	$OpenBSD: memmove.c,v 1.3 2017/11/29 05:13:57 guenther Exp $ */
 /*-
  * Copyright (c) 1990 The Regents of the University of California.
  * All rights reserved.
@@ -110,4 +110,4 @@ memmove(void *dst0, const void *src0, size_t length)
 done:
 	return (dst0);
 }
-DEF_STRONG(memmove);
+DEF_BUILTIN(memmove);

src/lib/libc/string/memset.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: memset.c,v 1.7 2015/08/31 02:53:57 guenther Exp $ */
+/*	$OpenBSD: memset.c,v 1.8 2017/11/29 05:13:57 guenther Exp $ */
 /*-
  * Copyright (c) 1990 The Regents of the University of California.
  * All rights reserved.
@@ -45,4 +45,4 @@ memset(void *dst, int c, size_t n)
 	}
 	return (dst);
 }
-DEF_STRONG(memset);
+DEF_BUILTIN(memset);