Make elf_aux_info() as public libc function.

- Teach elf aux vector functions about newly added AT_HWCAP and AT_HWCAP2
  vectors.
- Export _elf_aux_info() as new public libc function elf_aux_info(3)

The elf_aux_info(3) should be considered as FreeBSD counterpart of glibc
getauxval() with more robust interface.

Note:
We cannot name this new function as getauxval(), with glibc compatible
interface. Some ports autodetect its existence and then expects that all
Linux specific AT_<*> vectors are defined and implemented.

MFC after:	1 month
Reviewed by:	kib
Differential Revision:	https://reviews.freebsd.org/D12743

lib/libc/gen/Symbol.map

@@ -398,6 +398,7 @@ FBSD_1.5 {
 	devname;
 	devname_r;
 	dirname;
+	elf_aux_info;
 	fts_children;
 	fts_close;
 	fts_get_clientptr;

lib/libc/gen/auxv.c

@@ -33,6 +33,7 @@ __FBSDID("$FreeBSD$");
 #include <link.h>
 #include <pthread.h>
 #include <string.h>
+#include <sys/auxv.h>
 #include "un-namespace.h"
 #include "libc_private.h"
 
@@ -65,8 +66,10 @@ __init_elf_aux_vector(void)
 
 static pthread_once_t aux_once = PTHREAD_ONCE_INIT;
 static int pagesize, osreldate, canary_len, ncpus, pagesizes_len;
+static int hwcap_present, hwcap2_present;
 static char *canary, *pagesizes;
 static void *timekeep;
+static u_long hwcap, hwcap2;
 
 static void
 init_aux(void)
@@ -83,6 +86,16 @@ init_aux(void)
 			canary_len = aux->a_un.a_val;
 			break;
 
+		case AT_HWCAP:
+			hwcap_present = 1;
+			hwcap = (u_long)(aux->a_un.a_val);
+			break;
+
+		case AT_HWCAP2:
+			hwcap2_present = 1;
+			hwcap2 = (u_long)(aux->a_un.a_val);
+			break;
+
 		case AT_PAGESIZES:
 			pagesizes = (char *)(aux->a_un.a_ptr);
 			break;
@@ -110,6 +123,8 @@ init_aux(void)
 	}
 }
 
+__weak_reference(_elf_aux_info, elf_aux_info);
+
 int
 _elf_aux_info(int aux, void *buf, int buflen)
 {
@@ -130,14 +145,27 @@ _elf_aux_info(int aux, void *buf, int buflen)
 		} else
 			res = ENOENT;
 		break;
+	case AT_HWCAP:
+		if (hwcap_present && buflen == sizeof(u_long)) {
+			*(u_long *)buf = hwcap;
+			res = 0;
+		} else
+			res = ENOENT;
+		break;
+	case AT_HWCAP2:
+		if (hwcap2_present && buflen == sizeof(u_long)) {
+			*(u_long *)buf = hwcap2;
+			res = 0;
+		} else
+			res = ENOENT;
+		break;
 	case AT_PAGESIZES:
 		if (pagesizes != NULL && pagesizes_len >= buflen) {
 			memcpy(buf, pagesizes, buflen);
 			res = 0;
 		} else
 			res = ENOENT;
 		break;
-
 	case AT_PAGESZ:
 		if (buflen == sizeof(int)) {
 			if (pagesize != 0) {

sys/sys/auxv.h

@@ -0,0 +1,37 @@
+/*-
+ * Copyright (c) 2017 Michal Meloun
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ *
+ * $FreeBSD$
+ */
+
+#ifndef	_SYS_AUXV_H_
+#define	_SYS_AUXV_H_
+
+#include <sys/types.h>
+#include <machine/elf.h>
+
+int elf_aux_info(int aux, void *buf, int buflen);
+
+#endif /* !_SYS_AUXV_H_ */