syscall: provide Renameat as a wrapper around renameat2 on riscv64

linux/riscv64 does not provide a renameat syscall and there is no SYS_RENAMEAT
defined in the syscall package. Provide Renameat as a wrapper around renameat2
instead.

Updates #27532

Change-Id: I4864bdf0055e4272920583dcfb0efe2fd40adcbc
Reviewed-on: https://go-review.googlesource.com/c/go/+/212762


Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>