syscall: add support for freebsd/riscv64

syscall: add support for freebsd/riscv64

Updates #53466

Change-Id: I3c156400a105e711d6da0980854c08bf2f7e415a
Reviewed-on: https://go-review.googlesource.com/c/go/+/431655
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Mikaël Urankar <mikael.urankar@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Joel Sing <joel@sing.id.au>
Reviewed-by: Dmitri Goutnik <dgoutnik@gmail.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>