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superseded by renameat2.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_amd64.go b/unix/syscall_linux_amd64.go
index 615f291..87a3074 100644
--- a/unix/syscall_linux_amd64.go
+++ b/unix/syscall_linux_amd64.go
@@ -43,6 +43,7 @@
 //sys	Pause() (err error)
 //sys	Pread(fd int, p []byte, offset int64) (n int, err error) = SYS_PREAD64
 //sys	Pwrite(fd int, p []byte, offset int64) (n int, err error) = SYS_PWRITE64
+//sys	Renameat(olddirfd int, oldpath string, newdirfd int, newpath string) (err error)
 //sys	Seek(fd int, offset int64, whence int) (off int64, err error) = SYS_LSEEK
 
 func Select(nfd int, r *FdSet, w *FdSet, e *FdSet, timeout *Timeval) (n int, err error) {