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/uapi/asm/auxvec.h

@@ -15,6 +15,7 @@
 #ifndef _ASM_TILE_AUXVEC_H
 #define _ASM_TILE_AUXVEC_H
 
-/* No extensions to auxvec */
+/* The vDSO location. */
+#define AT_SYSINFO_EHDR         33
 
 #endif /* _ASM_TILE_AUXVEC_H */