arm: add vdso support

vdso will be available on arm in linux v4.2, the user-space code
for it is in kernel commit 8512287a8165592466cb9cb347ba94892e9c56a5

arch/arm/syscall_arch.h

@@ -72,3 +72,7 @@ static inline long __syscall6(long n, long a, long b, long c, long d, long e, lo
 	register long r5 __asm__("r5") = f;
 	__asm_syscall("r"(r7), "0"(r0), "r"(r1), "r"(r2), "r"(r3), "r"(r4), "r"(r5));
 }
+
+#define VDSO_USEFUL
+#define VDSO_CGT_SYM "__vdso_clock_gettime"
+#define VDSO_CGT_VER "LINUX_2.6"