unix: use Renameat2 to implement Renameat on linux/riscv64

On linux/riscv64, the renameat syscall no longer exists and has been
superseded by renameat2. Use Renameat2 to implement Renameat on
linux/riscv64.

Based on a patch by Ryan O'Leary.

Updates golang/go#27532

Change-Id: I7c60c7f824750408265f67397b8ac1fa7226f425
Reviewed-on: https://go-review.googlesource.com/c/157899
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_linux_riscv64.go

@@ -207,3 +207,7 @@ func Poll(fds []PollFd, timeout int) (n int, err error) {
 	}
 	return ppoll(&fds[0], len(fds), ts, nil)
 }
+
+func Renameat(olddirfd int, oldpath string, newdirfd int, newpath string) (err error) {
+	return Renameat2(olddirfd, oldpath, newdirfd, newpath, 0)
+}