[dev.simd] internal/buildcfg: disable SIMD experiment by default for merge to master

Change-Id: Id3f601d2010d896992ef36a337f2ef6305921110
Reviewed-on: https://go-review.googlesource.com/c/go/+/783022
Commit-Queue: Cherry Mui <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Bypass: Cherry Mui <cherryyz@google.com>