syscall: export X__ifi_pad and X_f on s390x and riscv64

syscall: export X__ifi_pad and X_f on s390x and riscv64

This brings the exported field names on these GOARCHes
in line with the other supported linux platforms.

Fixes #57313.

Change-Id: I2107a7116ba60dc0c0a27f776318e3eb69e51682
Reviewed-on: https://go-review.googlesource.com/c/go/+/457557
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>