[dev.simd] internal/buildcfg: disable simd experiment for reverse merge

Change-Id: I883e800d85b95d39635845ed1eed53eb4a78c63b
Reviewed-on: https://go-review.googlesource.com/c/go/+/784922
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>