internal/buildcfg: enable specializedmalloc experiment

internal/buildcfg: enable specializedmalloc experiment

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64_c2s16-perf_vs_parent,gotip-linux-amd64_c3h88-perf_vs_parent,gotip-linux-arm64_c4ah72-perf_vs_parent,gotip-linux-arm64_c4as16-perf_vs_parent
Change-Id: I6a6a6964c4c596bbf4f072b5a44a34c3ce4f6541
Reviewed-on: https://go-review.googlesource.com/c/go/+/696536
Reviewed-by: Michael Matloob <matloob@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>