cmd/compile, runtime: guard X15 zeroing with GOEXPERIMENT=simd

cmd/compile, runtime: guard X15 zeroing with GOEXPERIMENT=simd

If simd experiment is not enabled, the compiler doesn't use the
AVX part of the register. So only zero it with the SSE instruction.

Change-Id: Ia3bdf34a9ed273128db2ee0f4f5db6f7cc76a975
Reviewed-on: https://go-review.googlesource.com/c/go/+/724720
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>