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 <joel@sing.id.au>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Joel Sing <joel@sing.id.au>