syscall: resync various riscv64 structs with x/sys/unix

For some reason the existing versions have additional padding - manually resync these
structs with those in x/sys/unix, also bringing in pollFd which does not currently
exist for linux/riscv64.

Cross checked with the output from 'go tool cgo -godefs' on a linux/riscv64 machine.

Updates #27532

Change-Id: I9fd2d3ad7222208a138773f4a9153c3c9cad7381
Reviewed-on: https://go-review.googlesource.com/c/go/+/212763


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