runtime: prevent TLS fetching instructions from being assembled on NaCl/ARM
runtime: prevent TLS fetching instructions from being assembled on NaCl/ARM

They are dead code already, but the verifier is still not happy.
Don't assemble them at all.

Looks like it has been like that for long. I don't know why it
was ok. Maybe the verifier is now more picky?

Fixes #19884.

Change-Id: Ib806fb73ca469789dec56f52d484cf8baf7a245c
Reviewed-on: https://go-review.googlesource.com/40111
Run-TryBot: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
Reviewed-by: Dave Cheney <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@cheney.net>

josharian/go,vsekhar/elastic-go,polyverse-security/go,Cofyc/go,golang/go,akutz/go,Cofyc/go,CAFxX/go,vsekhar/elastic-go,polyverse-security/go,ImJasonH/go,codestation/go,derekmarcotte/go,ImJasonH/go,josharian/go,cloudflare/go,golang/go,akutz/go,christopher-henderson/Go,Cofyc/go,vsekhar/elastic-go,nawawi/go,vsekhar/elastic-go,derekmarcotte/go,cloudflare/go,cloudflare/go,Cofyc/go,codestation/go,polyverse-security/go,josharian/go,codestation/go,CAFxX/go,nawawi/go,nawawi/go,ImJasonH/go,akutz/go,cloudflare/go,golang/go,golang/go,ImJasonH/go,golang/go,polyverse-security/go,codestation/go,derekmarcotte/go,ImJasonH/go,vsekhar/elastic-go,Cofyc/go,CAFxX/go,vsekhar/elastic-go,codestation/go,josharian/go,golang/go,ImJasonH/go,cloudflare/go,ImJasonH/go,CAFxX/go,nawawi/go,vsekhar/elastic-go,akutz/go,cloudflare/go,josharian/go,polyverse-security/go,polyverse-security/go,Cofyc/go,Cofyc/go,nawawi/go,polyverse-security/go,cloudflare/go,codestation/go,akutz/go,CAFxX/go,derekmarcotte/go,nawawi/go,Cofyc/go,josharian/go,CAFxX/go,vsekhar/elastic-go,cloudflare/go,akutz/go,CAFxX/go,golang/go,derekmarcotte/go,derekmarcotte/go,derekmarcotte/go,akutz/go,akutz/go,codestation/go,codestation/go,polyverse-security/go,golang/go,nawawi/go,josharian/go,nawawi/go,ImJasonH/go,josharian/go,derekmarcotte/go,CAFxX/go