mips: add vdso support

vdso support is available on mips starting with kernel 4.4, see kernel
commit a7f4df4e21 "MIPS: VDSO: Add implementations of gettimeofday()
and clock_gettime()" for details.

In Linux kernel 4.4.0 the mips code returns -ENOSYS in case it can not
handle the vdso call and assumes the libc will call the original
syscall in this case. Handle this case in musl. Currently Linux kernel
4.4.0 handles the following types: CLOCK_REALTIME_COARSE,
CLOCK_MONOTONIC_COARSE, CLOCK_REALTIME and CLOCK_MONOTONIC.

arch/mips/syscall_arch.h

@@ -161,3 +161,7 @@ static inline long __syscall6(long n, long a, long b, long c, long d, long e, lo
 	if (n == SYS_fstatat) __stat_fix(c);
 	return r2;
 }
+
+#define VDSO_USEFUL
+#define VDSO_CGT_SYM "__vdso_clock_gettime"
+#define VDSO_CGT_VER "LINUX_2.6"