CMOVs were not introduced until P6.  We need 386 to run on
Pentium MMX.

Fixes #13923

Change-Id: Iee9572cd83e64c3a1336bc1e6b300b048fbcc996
Reviewed-on: https://go-review.googlesource.com/18621Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>