ARM: VDSO support

Beginning with the upcoming 4.1 release, Linux on a subset of 32-bit
ARM hardware will provide fast user-space implementations of the
following system calls:

- gettimeofday
- clock_gettime

The kernel implementation depends on the ARMv7 Generic Timers
Extension to accelerate these system calls.  So CPUs such as
Cortex-A15 and -A7 benefit, while Cortex-A9, -A8, and pre-v7 CPUs do
not.  On systems where the VDSO does not provide any speedup, the
kernel prevents the relevant symbol lookups from succeeding.

On OMAP5 (Cortex-A15) gettimeofday latency decreases from ~350ns to
~120ns.  On BeagleBone Black (Cortex-A8) it goes from ~650ns to
~660ns, which to my mind is an acceptable cost.

Verified that no new test failures are introduced on kernels with and
without the VDSO.

	* sysdeps/unix/sysv/linux/arm/Makefile: (sysdep_routines):
	Include dl-vdso.
	* sysdeps/unix/sysv/linux/arm/init-first.c: New file:
	Use VDSO routines for gettimeofday, clock_gettime if
	available.
	* sysdeps/unix/sysv/linux/arm/libc-vdso.h: New file:
	Declare VDSO symbols.
	* sysdeps/unix/sysv/linux/arm/sysdep.h:
	[HAVE_GETTIMEOFDAY_VSYSCALL]: Define.
	[HAVE_CLOCK_GETTIME_VSYSCALL]: Define.
	* sysdeps/unix/sysv/linux/arm/Versions: Add
	__vdso_clock_gettime.

sysdeps/unix/sysv/linux/arm/libc-vdso.h

@@ -0,0 +1,33 @@
+/* VDSO function pointer declarations.  Linux/ARM.
+   Copyright (C) 2015 Free Software Foundation, Inc.
+
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library.  If not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#ifndef _LIBC_VDSO_H
+#define _LIBC_VDSO_H
+
+#ifdef SHARED
+
+# include <sysdep-vdso.h>
+
+extern int (*VDSO_SYMBOL(gettimeofday)) (struct timeval *, void *)
+   attribute_hidden;
+extern int (*VDSO_SYMBOL(clock_gettime)) (clockid_t, struct timespec *);
+
+#endif
+
+#endif /* _LIBC_VDSO_H */