tile: implement gettimeofday() via vDSO

This change creates the framework for vDSO calls, makes the existing
rt_sigreturn() mechanism use it, and adds a fast gettimeofday().
Now that we need to expose the vDSO address to userspace, we add
AT_SYSINFO_EHDR to the set of aux entries provided to userspace.
(You can disable any extra vDSO support by booting with vdso=0,
but the rt_sigreturn vDSO page will still be provided.)

Note that glibc has supported the tile vDSO since release 2.17.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>

arch/tile/include/asm/mmu.h

@@ -22,6 +22,7 @@ struct mm_context {
 	 * semaphore but atomically, but it is conservatively set.
 	 */
 	unsigned long priority_cached;
+	unsigned long vdso_base;
 };
 
 typedef struct mm_context mm_context_t;