cmd/asm: report an error when trying to do spectre on 386
cmd/asm: report an error when trying to do spectre on 386

The compiler refuses to do spectre mitigation on 386, but the
assembler doesn't. Fix that.

Fixes #49006

Change-Id: I887b6f7ed7523a47f463706f06ca4c2c6e828b6b
Reviewed-on: https://go-review.googlesource.com/c/go/+/356190
Trust: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
Run-TryBot: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

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