RISC-V: Add VDSO entries for clock_get/gettimeofday/getcpu

For now these are just placeholders that execute the syscall.  We will
later optimize them to avoid kernel crossings, but we'd like to have the
VDSO entries from the first released kernel version to make the ABI
simpler.

Signed-off-by: Andrew Waterman <andrew@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

arch/riscv/kernel/vdso/clock_gettime.S

@@ -0,0 +1,26 @@
+/*
+ * Copyright (C) 2017 SiFive
+ *
+ *   This program is free software; you can redistribute it and/or
+ *   modify it under the terms of the GNU General Public License
+ *   as published by the Free Software Foundation, version 2.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *   GNU General Public License for more details.
+ */
+
+#include <linux/linkage.h>
+#include <asm/unistd.h>
+
+	.text
+/* int __vdso_clock_gettime(clockid_t clock_id, struct timespec *tp); */
+ENTRY(__vdso_clock_gettime)
+	.cfi_startproc
+	/* For now, just do the syscall. */
+	li a7, __NR_clock_gettime
+	ecall
+	ret
+	.cfi_endproc
+ENDPROC(__vdso_clock_gettime)