syscall: support generating netbsd/arm64 files in mkall.sh

syscall: support generating netbsd/arm64 files in mkall.sh

CL 155739 added the generated files but didn't update mkall.sh. Do so
now.

Updates #30824

Change-Id: I642bbff6afbc976091a0dc291fa2beff5e245246
Reviewed-on: https://go-review.googlesource.com/c/go/+/175237
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Run-TryBot: Benny Siegert <bsiegert@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>