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superseded by renameat2.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>
diff --git a/unix/syscall_linux_arm.go b/unix/syscall_linux_arm.go
index ad2bd25..cda3559 100644
--- a/unix/syscall_linux_arm.go
+++ b/unix/syscall_linux_arm.go
@@ -89,6 +89,7 @@
 //sys	Listen(s int, n int) (err error)
 //sys	Lstat(path string, stat *Stat_t) (err error) = SYS_LSTAT64
 //sys	Pause() (err error)
+//sys	Renameat(olddirfd int, oldpath string, newdirfd int, newpath string) (err error)
 //sys	sendfile(outfd int, infd int, offset *int64, count int) (written int, err error) = SYS_SENDFILE64
 //sys	Select(nfd int, r *FdSet, w *FdSet, e *FdSet, timeout *Timeval) (n int, err error) = SYS__NEWSELECT
 //sys	Setfsgid(gid int) (err error) = SYS_SETFSGID32