ARM: VDSO support

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.