runtime: work around vet asmdecl checks for openbsd/386 mstart_stub
runtime: work around vet asmdecl checks for openbsd/386 mstart_stub

Include a NOP with the SP in order to disable/bypass vet asmdecl checks
for runtime.mstart_stub on openbsd/386. Without this we get:

runtime/sys_openbsd_386.s:33:1: [386] mstart_stub: use of 32(SP) points beyond argument frame

Change-Id: I834ae3dbceffcb5776481b076ec2afe3700671cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/315789
Trust: Joel Sing <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@sing.id.au>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Run-TryBot: Joel Sing <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@sing.id.au>

CAFxX/go,golang/go,cloudflare/go,CAFxX/go,cloudflare/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,nawawi/go,golang/go,cloudflare/go,nawawi/go,golang/go,golang/go,CAFxX/go,CAFxX/go,cloudflare/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,nawawi/go,golang/go,cloudflare/go,nawawi/go