We insert guard instructions after each "dangerous" instruction
to make NaCl's validator happy. This happens before asmout. If
in asmout an instruction is split to two dangerous instructions,
but only one guard instruction is inserted, the validation fails.
Therefore don't split instructions on NaCl.

Fixes #20595.

Change-Id: Ie34f209bc7d907d6d16ecef6721f88420981ac01
Reviewed-on: https://go-review.googlesource.com/45021
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>