plan9: modify mkall.sh to support zsyscall*.go files generation
plan9: modify mkall.sh to support zsyscall*.go files generation

Modify mkall.sh to support generation of zsyscall_plan9_{386,amd64,arm}.go
files. This does not generate any git diff as they were manually generated
with the same commands.

Fixes golang/go#29586

Change-Id: I5316fdb46f306974584eee653b439408294ea1cf
Reviewed-on: https://go-review.googlesource.com/c/sys/+/179041
Reviewed-by: David du Colombier <ec8ae1e6620d4bab5e0e7aa2a6ae41aad63e3708@gmail.com>
Reviewed-by: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>

multiplay/golang-sys,golang/sys,golang/sys,multiplay/golang-sys,golang/sys,multiplay/golang-sys,multiplay/golang-sys