syscall: use libc in Exec on openbsd/arm64

Like on openbsd/amd64, use libc instead of direct syscalls on
openbsd/arm64.

This was likely missed in CL 286815. A similar change was done for
openbsd/amd64 in CL 270380.

Updates #36435

Change-Id: Ie496a6130f1a43d30974502777db12217e65c551
Reviewed-on: https://go-review.googlesource.com/c/go/+/312229


Trust: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Joel Sing <joel@sing.id.au>